Partial response receiver

ABSTRACT

A receive circuit for receiving a signal transmitted via an electric signal conductor. A first sampling circuit generates a first sample value that indicates whether the signal exceeds a first threshold level, and a second sampling circuit generates a second sample value that indicates whether the signal exceeds a second threshold level. A first select circuit receives the first and second sample values from the first and second sampling circuits and selects, according to a previously generated sample value, either the first sample value or the second sample value to be output as a selected sample value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 10/662,872, filed Sep. 16, 2003, which claims priority to U.S.Provisional Application No. 60/461,729, filed Apr. 9, 2003, both ofwhich are hereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of communications,and more particularly to high speed electronic signaling within andbetween integrated circuit devices.

BACKGROUND

Electrical pulses transmitted on a band-limited signaling path dispersein time as they travel from source to destination. In systems in whichdata is transmitted as a sequence of level-encoded electrical pulses,such time-domain dispersion results in a blending of neighboring pulses;an effect known as dispersion-type inter-symbol interference (ISI).Dispersion-type ISI becomes more pronounced at faster signaling rates,ultimately degrading the signal quality to the point at whichdistinctions between originally transmitted signal levels may be lost.

FIG. 1 illustrates a prior-art signaling system having an equalizingoutput driver 109 and an equalizing receiver 103 to mitigatedispersion-type ISI resulting from signal transmission on a signalingpath 102. The receiver 103 includes a sampling circuit 105 to generatedigitized samples 106 of the incoming signal, a shift register 107 tostore some number (N) of the most recently received samples, and anequalizer 112 to generate an equalization signal 114 based on samplesstored in the shift register 107. Ideally, the equalization signal 114represents the residual signal level on path 102 of the N prior receivedsamples in the incoming signal so that, by subtracting the equalizationsignal 114 from the incoming signal in difference circuit 115, thedispersion-type ISI resulting from the prior transmissions is canceled.Because the prior decisions of the sampling circuit 105 are fed back tothe sampling circuit input in the form of the equalization signal 114,the receiver 103 is commonly referred to as a decision feedbackequalizer (DFE).

One major limitation of the DFE 103 is that the time delay in theoverall feedback path from sampling circuit 105 to difference circuit115 makes it difficult to generate the equalization feedback signal 114in time to equalize the signal level of the immediately following datavalue if the least latent sample (i.e., the most recently capturedsample 106) is included in the equalization feedback signal 114.Including the least latent sample in the equalization signal isparticularly challenging in modern high-speed signaling systems in whichincoming symbols are present on the signal path 102 for extremely briefintervals (e.g., less than a nanosecond for signal rates above oneGigabit per second). One solution to the least-latent sample problem isto omit one or more of the least-latent samples from contributing togeneration of the decision-feedback equalization signal. Unfortunately,the least latent sample, being nearest in time to the incoming symbol,tends to be the largest contributor to dispersion-type ISI and thereforea primary objective of cancellation by the DFE. Consequently, insignaling systems in which the least-latent sample is omitted fromcontribution to decision-feedback equalization, transmit-sidepre-emphasis is often used to decrease the dispersion-type ISI caused bythe least-latent symbol. That is, when a given symbol is transmitted bythe equalizing output driver 109, one or more previously transmittedsymbols stored in shift register 113 (i.e., the least latent symbolsrelative to the outgoing symbol) are used to pre-shape the outgoingwaveform to reduce the dispersion-type ISI observed at the receiver.Unfortunately, as can be seen in the raw and equalized pulse responsesdepicted in FIG. 2, forcing the least-latent sample, D_(N−1), to zero(or near zero) results in significant attenuation of the overall signallevel, thereby reducing signaling margins and ultimately limiting thedata rate of the signaling system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 illustrates a prior-art signaling system;

FIG. 2 illustrates raw and equalized pulse responses observed in theprior art signaling system of FIG. 1;

FIG. 3 illustrates a signaling system that includes a partial responsereceiver according to an embodiment of the invention;

FIG. 4 illustrates a relationship between clock and data signals in oneembodiment of the signaling system of FIG. 3;

FIG. 5 illustrates the bimodal distribution of a binary signal observedat a signal receiver when the primary source of dispersion-type ISI isthe signal transmitted in the immediately preceding symbol time;

FIG. 6 illustrates the four partial response signal levels depicted inFIG. 5 relative to a common mode level, L_(CM);

FIG. 7 illustrates a partial response receive circuit according to anembodiment of the invention;

FIG. 8 illustrates a double data rate signaling protocol in which twosymbols are transmitted in succession during each cycle of a samplingclock signal;

FIG. 9 illustrates a partial response receive circuit for use in adouble data rate signaling system;

FIG. 10 illustrates even and odd pipelines of sample values generatedwithin the partial response receive circuit of FIG. 9;

FIG. 11 illustrates the use of embedded scoping to generate a time-basedtrace of an incoming data signal;

FIG. 12 illustrates a signaling system according to an embodiment of theinvention;

FIG. 13 illustrates a waveform trace of a pulse response captured by anembedded scope within the signaling system of FIG. 12;

FIG. 14 illustrates a partial response receiver according to anembodiment of the invention that adaptively generates partial responsethreshold levels;

FIG. 15 illustrates a embodiment of the adaptive module of FIG. 14;

FIG. 16 illustrates an alternative circuit arrangement that may be usedin place of the averaging circuits of FIG. 15;

FIG. 17 illustrates a comparator that may be used within the receivercircuits and level sampling circuits of FIGS. 7, 9 and 14;

FIG. 18 illustrates an embodiment of the current DAC of FIG. 17;

FIG. 19 illustrates an alternative comparator embodiment that may beused within the receiver circuits and level sampling circuits of FIGS.7, 9 and 14;

FIG. 20 illustrates a partial response receiver according to anotheralternative embodiment of the invention;

FIG. 21 illustrates the operation of the adaptive module of FIG. 20according an embodiment of the invention;

FIG. 22 illustrates an embodiment of a dual mode receiver that may beoperated in either a multi-level signaling mode or a partial responsemode;

FIG. 23 illustrates a multi-level signaling protocol used within dualmode receiver of FIG. 22;

FIG. 24 illustrates a partial response receiver having a clock datarecovery function according to an embodiment of the invention;

FIG. 25 illustrates a number of possible data signal transitions whenthe incoming data signal has the bimodal distribution shown in FIG. 4;

FIG. 26 illustrates an embodiment of a clock recovery circuit thatadjusts the phase of edge clock signal and sampling clock signal basedon selected transitions detected in the incoming data signal;

FIG. 27 illustrates a potential source of non-convergence in a systemthat adaptively generates partial response threshold levels;

FIG. 28 illustrates an embodiment of a dual-mode, partial responsereceiver with clock data recovery;

FIG. 29 illustrates signal transitions between successive 4-PAM (PulseAmplitude Modulation) symbols;

FIG. 30 illustrates data signal transitions that may be used for clockrecovery when the dual mode receiver of FIG. 28 is operated in a partialresponse mode;

FIG. 31 illustrates a transition detect circuit and sample selectcircuit that may be used within the clock recovery circuit of FIG. 28;

FIG. 32 illustrates additional data signal transitions that may be usedfor clock recovery when the dual mode receiver of FIG. 28 is operated ina partial response mode;

FIG. 33 illustrates a threshold select circuit according to anembodiment of the invention;

FIG. 34 illustrates a distribution 770 of signal levels in a system inwhich the two most recently received symbols are the primary source ofresidual channel response;

FIG. 35 illustrates a partial response receiver according to anembodiment of the invention that operates in accordance with the partialresponse states shown in FIG. 34;

FIG. 36 illustrates a partial response receiver according to anotherembodiment of the invention;

FIG. 37 illustrates a constellation of possible signal levels in a 4-PAMsignaling system;

FIG. 38 illustrates a 4-PAM partial response receiver according to anembodiment of the invention;

FIG. 39 illustrates an input circuit that includes multiple partialresponse receivers according to an embodiment of the invention;

FIG. 40 illustrates an input circuit according to an alternativeembodiment of the invention;

FIG. 41 illustrates a partial-response signaling system according to anembodiment of the invention;

FIG. 42 illustrates a raw pulse response and a pulse response equalizedby the signaling system of FIG. 41 to enable partial-response datareception; and

FIG. 43 illustrates residual inter-symbol interference that may beequalized by transmit preemphasis within the signaling system of FIG.41.

DETAILED DESCRIPTION

In the following description and in the accompanying drawings, specificterminology and drawing symbols are set forth to provide a thoroughunderstanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, the interconnection betweencircuit elements or circuit blocks may be shown or described asmulti-conductor or single conductor signal lines. Each of themulti-conductor signal lines may alternatively be single-conductorsignal lines, and each of the single-conductor signal lines mayalternatively be multi-conductor signal lines. Signals and signalingpaths shown or described as being single-ended may also be differential,and vice-versa. Similarly, signals described or depicted as havingactive-high or active-low logic levels may have opposite logic levels inalternative embodiments. As another example, circuits described ordepicted as including metal oxide semiconductor (MOS) transistors mayalternatively be implemented using bipolar technology or any othertechnology in which a signal-controlled current flow may be achieved.With respect to terminology, a signal is said to be “asserted” when thesignal is driven to a low or high logic state (or charged to a highlogic state or discharged to a low logic state) to indicate a particularcondition. Conversely, a signal is said to be “deasserted” to indicatethat the signal is driven (or charged or discharged) to a state otherthan the asserted state (including a high or low logic state, or thefloating state that may occur when the signal driving circuit istransitioned to a high impedance condition, such as an open drain oropen collector condition). A signal driving circuit is said to “output”a signal to a signal receiving circuit when the signal driving circuitasserts (or deasserts, if explicitly stated or indicated by context) thesignal on a signal line coupled between the signal driving and signalreceiving circuits. A signal line is said to be “activated” when asignal is asserted on the signal line, and “deactivated” when the signalis deasserted. Additionally, the prefix symbol “/” attached to signalnames indicates that the signal is an active low signal (i.e., theasserted state is a logic low state). A line over a signal name (e.g., ‘<signal name>’) is also used to indicate an active low signal.

A novel signal receiving circuit for use in electrical signaling systemsis disclosed in various embodiments. In one embodiment, the signalreceiving circuit includes a pair of comparators for comparing anincoming electrical signal against respective threshold levels, witheach threshold level being set according to the level of a dispersedcomponent of a previously received signal. More specifically, one of thethreshold levels corresponds to the dispersion-type ISI introduced by apreviously transmitted signal representative of a first digital state,and the other of the threshold levels corresponds to the dispersion-typeISI introduced when the previously transmitted signal is representativeof a second digital state. In the case of a binary signaling system, thepreviously transmitted signal is resolved into one of two states by thereceiving circuit, then used to select which of the comparators is tosource the sample to be used for selection of the subsequent pair ofsamples. By this arrangement, each of the comparators compares theincoming signal against a threshold that is offset from a nominal valueaccording to a respective, anticipated level of ISI, with the comparatorhaving its threshold level offset in the direction that corresponds tothe state of the previously received signal ultimately being selected asthe sample source. Because samples are captured on the assumption ofdispersion-type ISI resulting from both possible states of thepreviously captured sample (i.e., the least-latent sample), both samplesare available for later selection when the state of the previouslycaptured sample is resolved. Thus, by establishing separate thresholdlevels according to the different possible levels of dispersion-type ISIand comparing the incoming signal against each threshold level, thetiming problems associated with generating an equalization signal thatincludes the least latent sample are avoided. Because the thresholdlevel supplied to each of the comparators includes an offset accordingto the residual, partial response of the signaling path to thepreviously transmitted signal, a receive circuit having suchoffset-threshold comparators is referred to herein as a partial responsereceiver.

In another embodiment of the invention, a multi-level signal receiver(i.e., receiver capable of receiving a symbol representative of morethan a single binary bit), may be switched to a binary signaling,partial response mode. In one implementation, a multi-PAM receiver(Pulse Amplitude Modulation receiver—a type of multi-level signalreceiver) includes multiple comparators for distinguishing differentpossible levels (e.g., 4 levels in a 4-PAM receiver, 8 levels in an8-PAM receiver, etc.) such that no additional comparators are necessaryto support partial response operation. Instead, when the partialresponse mode is enabled, the threshold levels supplied to thecomparators are adjusted to match the partial response levels for theleast-latent symbol (or set of N least-latent symbols) and the postprocessing of the comparator outputs is switched from a symbol decodingoperation to a sample selection operation.

In other embodiments of the invention, methods and circuits forgenerating partial response threshold levels are disclosed. Also,methods and circuits for transmitting a partial-response data signalthat includes a partial response from selected prior-transmitted bitsare disclosed. Further, methods and circuits for recovering a clocksignal from a partial response data signal are also disclosed.

Signaling System Overview

FIG. 3 illustrates a signaling system 117 in which embodiments of thepresent invention may be used. The system 117 includes an equalizingtransmitter 118 and equalizing receiver 116 coupled to one another via ahigh-speed signal path 122, and a controller 141 coupled to thetransmitter 118 and the receiver 116 via relatively low-speed signalpaths 142A and 142B, respectively. In one embodiment, the signal path122 is formed by component signal paths 122A, 122B and 122C (e.g.,transmission lines that introduce respective, nonzero propagation delaysand exhibit respective impedance characteristics), each disposed onrespective circuit boards that are coupled to one another via circuitboard interfaces 125 and 127 (e.g., connectors). In a specificimplementation, signal path 122B is formed on a backplane and signalpaths 122A and 122C are formed on respective daughterboards (e.g., linecards) that are removably coupled to the backplane via circuit boardinterfaces 125 and 127. The transmitter 118 and receiver 116 areimplemented in respective integrated circuit (IC) devices that aremounted on the daughterboards. The controller, which may be a general orspecial purpose processor, state machine or other logic circuit, isimplemented within a third integrated circuit device mounted to a yetanother circuit board. In the embodiment of FIG. 3, signal paths 142Aand 142B are used to convey configuration information from thecontroller 141 to the transmitter 118 and receiver 116, respectively,and may be disposed on the same circuit board (or circuit boards) assignal path 122 or implemented by an alternative structure such as acable. The controller may alternatively be coupled to the transmitter118 and receiver 116 by a shared signal path such as a multi-drop bus.The operation of the controller 141 is discussed in greater detailbelow. In alternative embodiments, the IC devices containing thetransmitter 118, receiver 116 and controller 141 may be mounted to acommon structure with the signaling paths 122, 142A and 142B coupleddirectly to the IC devices (e.g., all three ICs mounted to a circuitboard and coupled to one another via circuit board traces, or all threeICs packaged within a single multi-chip module with signal paths 122 and142 formed between the ICs by bond wires or other conductingstructures). Also, the transmitter 118, receiver 116 and controller 141,or any subset thereof, may be included within the same IC device (e.g.,system on chip) and the signaling paths 122 and/or 142 implemented by ametal layer or other conducting structure within the IC device.

The equalizing transmitter 118 transmits data on the signaling path 122during successive time intervals, referred to herein as symbol times. Inone embodiment, illustrated by the timing diagram of FIG. 4, each symboltime, T_(S), corresponds to a half cycle of a transmit clock signal,TCLK, such that two data values (e.g., values A and B) are transmittedon signaling path 122 per transmit clock cycle. The transmitted datasignal arrives at the input of the equalizing receiver 116 afterpropagation time, T_(P), and is sampled by the receiver 116 in responseto edges of a receive clock signal, RCLK. The receive clock signal maybe received within the receive circuit via an external clock line, ormay be a recovered version of a reference clock signal (e.g., recoveredby a delay-locked loop or phase locked loop circuit). In otherembodiments, discussed below, the receive clock signal may be recoveredfrom the transmitted data signal. Still referring to FIG. 4, the receiveclock signal has a quadrature phase relation to data valid windows(i.e., data eyes) in the incoming data signal such that each sample iscaptured at the midpoint of a data eye. In alternative embodiments, thesampling instant may be skewed relative to data eye midpoints asnecessary to satisfy signal setup and hold time requirements in thereceiver 116. Also, more or fewer symbols may be transmitted per cycleof the transmit clock signal.

The equalizing transmitter 118 includes a transmit shift register 124,output driver 121 and transmit equalizer 129; the transmit equalizer 129itself including a shift register 120 and a bank of output drivers 131.At the start of each symbol time, the data value at the head (i.e.,output) of the transmit shift register 124, referred to herein as theprimary data value, is driven onto the signal path 122 by the outputdriver 121, and the transmit equalizer 129 simultaneously drives anequalizing signal onto the signal path 122. This type of equalization isreferred to herein as transmit preemphasis. In one embodiment, thesignal driven onto the signal path 122 by the output driver 121(referred to herein as the primary signal) is a multi-level signalhaving one of four possible states (e.g., defined by four distinctsignal ranges) and therefore constitutes a symbol representative of twobinary bits of information. In alternative embodiments, the primarysignal may have more or fewer possible states and therefore representmore or fewer than two binary bits. Also, the primary signal may besingle-ended or differential (an additional signal line is provided tocarry the complement signal in the differential case), and may be avoltage or current mode signal.

Each of the output drivers 131 within the transmit equalizer 129 formeither a pre-tap driver or post-tap driver according to whether thesource data value has already been transmitted (post-tap data) or is yetto be transmitted (pre-tap data). In the specific embodiment of FIG. 3,the equalizer includes N post-tap drivers sourced by data values withinthe shift register 120 and one pre-tap driver sourced by a data valuewithin the transmit shift register 124. Accordingly, the resultantequalizing signal driven onto the data path 122 will have a signal levelaccording to data values having symbol latencies of −1, 1, 2, . . . , N,where the symbol latency of a given data value refers to the number ofsymbol times by which transmission of the data value precedes thetransmission of the primary value. Different numbers of post-tap andpre-tap drivers may be provided in alternative embodiments, therebyallowing for equalization based on values having different symbollatencies.

Still referring to FIG. 3, the equalizing receiver 116 includes apartial-response receive circuit 123, buffer 132 (e.g., shift register),tap select circuit 128 and tap select logic 139. Data signals aresampled by the partial response receive circuit 123 to generatedigitized samples that are stored in the buffer 132 for eventual use byapplication logic (not shown). Because the buffered data is stored forat least a predetermined time and represents historical data up to apredetermined number of symbol latencies, the buffered data forms asource of post-tap data values that may be selected by the tap selectcircuit 128 to source equalizer taps in a receive-side equalizercircuit. Because a subset of buffered data values may be selectedaccording to the precise symbol latencies of reflections and otherhigh-latency distortions, a relatively small number of buffered datavalues may be selected to form receive-side equalization taps havinglatencies that match the latencies of the distortions. By thisarrangement, high latency distortions may be reduced by receive-sideequalization without dramatically increasing the parasitic capacitanceof the receiver (i.e., as would result from a large number ofreceive-side equalization taps). In one embodiment, the tap select logic139 is a configuration circuit that outputs a tap select signal 134according to a configuration value. The configuration value may beautomatically generated by system 117 (e.g., at system startup) or maybe empirically determined and stored within the configuration circuit orelsewhere within system 117.

In the embodiment of FIG. 3, buffer 132 is formed by a shift registerhaving a dead range component 133 having M storage elements and aselectable-range component 135 having R storage elements, the tap selectcircuit 128 being coupled to the selectable-range component 135 toselect the subset of tap data sources therefrom. In alternativeembodiments, the dead range component of the buffer 132 may includefewer than M storage elements or even zero storage elements, dependingon the time required to receive data and transfer data into the buffer132. Also, the tap select circuit 128 may be coupled to one or morestorage elements within the dead range component 133 to enable the sizeof the dead range to be programmed according to the configuration of thetransmit circuit 118. Finally, as discussed below, the buffer 132 mayinclude one or more parallel registers in addition to (or instead of)the shift register formed by components 133 and 135.

Still referring to FIG. 3, numerous alternative types of equalizationcircuits may be used within the receiver 116. For example, in oneembodiment, the receiver 116 includes an output driver 140 (illustratedin dashed outline in FIG. 3 to indicate its optional nature) to drive anequalizing signal onto the signal path 122 (and therefore to the inputof the partial response receive circuit 123) coincidentally with thesymbol time of an incoming signal. In another embodiment, the partialresponse receive circuit 123 includes a preamplifier having anequalizing subcircuit.

Multi-Modal Signal Distribution—Partial Response

FIG. 5 illustrates the bimodal distribution of a binary signal observedat a signal receiver when the primary source of dispersion-type ISI isthe signal transmitted in the immediately preceding symbol time. Thatis, the symbol sampled at time T_(N−1) is the primary source ofdispersion-type ISI in the symbol sampled at time T_(N); the symbolsampled at time T_(N) is the primary source of dispersion-type ISI inthe symbol sampled at time T_(N+1); and so forth. Referring to thesignal levels at time T_(N), it can be seen that if the if the precedingsymbol was a logic ‘1’, the partial response to the preceding symbolwill raise the signal level at time T_(N)to one of the circled levels‘11’ or ‘01’ (the second bit of the ‘11’ and ‘01’ bit sequencescorresponding to the state of the signal at time T_(N−1)). If thepreceding symbol was a logic ‘0’, the partial response to the precedingsymbol will lower the signal level at time T_(N) to one of thesquare-designated levels ‘10’ or ‘00’. In a single-ended signalingsystem, the incoming signal is sampled and compared with a referencevoltage level at the midpoint between the steady state extremes (i.e.,midway between the ‘11 . . . 1’ and ‘00 . . . 0’ signal levels).Accordingly, the worst case voltage margins (i.e., smallest voltagedifferences between the signal and reference voltage) occur when thesignal transitions from a ‘1’ to a ‘0’ or vice-versa; the ‘10’ and ‘01’signaling levels becoming indistinguishable when the partial responselevel (i.e., residual signal level from the preceding symboltransmission) is large. In a differential signaling system, the incomingsignal and its complement are sampled, for example, by a differentialamplifier which amplifies the difference between the complementarysignal levels. As in the single-ended case, the worst case voltagemargins occur when the differential signal transitions from a ‘1’ to a‘0’ or vice-versa; the ‘10’ and ‘01’ signaling levels becomingindistinguishable when the partial response pushes the ‘10’ and ‘01’levels to the common mode (i.e., the average of a differential signalpair).

FIG. 6 illustrates the four partial response signal levels depicted inFIG. 5 relative to a common mode level, L_(CM). In the case of adifferential signaling system, the partial response to the precedingsymbol may be viewed as increasing or decreasing the differentialamplitude (i.e., the amplitude difference between the signals that formthe differential signal pair) relative to a nominal differentialamplitude. Normalizing the positive and negative signal levels for thenominal differential amplitude to 1 and −1, respectively, the commonmode level, L_(CM), becomes zero, and the four possible signal levelsbecome 1+α, 1−α, −1+α and −1−α, where α represents the magnitude of thepartial response to the preceding symbol. Thus, when the precedingsymbol, D_(N−1), is a ‘0’, the incoming symbol, D_(N), is represented bya signal level at either 1−α or −1−α, depending on whether D_(N) is a‘1’ or ‘0’ Similarly, when D_(N−1)=1, the incoming symbol is representedby a signal level at either 1+α or −1+α according to the state of thecurrent symbol. In the former case (i.e., when D_(N−1)=0), the twocomplementary signal levels, −1−α and 1−α, have a common mode of −α, asshown in box 151. In the latter case (i.e., when D_(N−1)=1), the twocomplementary signal levels, 1+α and −1+α, have a common mode of +α, asshown in box 153.

Partial Response Receiver

FIG. 7 illustrates a partial response receive circuit 200 according toan embodiment of the invention that exploits the bi-modal characteristicof the signal levels depicted in FIGS. 5 and 6. The partial responsecircuitry includes a compare circuit 202 to sample the input datasignal, D_(N), and a decision circuit 204 to generate an output datavalue (i.e., received data value) based on data samples generated by thecompare circuit 202. Rather than a single comparator that distinguishesbetween binary signaling levels based on whether the sampled signal isabove or below a common mode threshold, the compare circuit 202 includestwo comparators 201 and 203 having threshold levels that are offset fromthe common mode threshold by the two possible partial responses to thepreceding symbol. That is, the threshold level of the comparator 201 isset the +α level, and the threshold level of the comparator 203 is setto the −α level. By this arrangement, if the preceding symbol was a ‘1’(i.e., D_(N−1)=1), the comparator 201 will resolve the incoming signalas being a ‘1’ or ‘0’ by determining whether the signal level is aboveor below the partial response level, +α. Conversely, if D_(N−1)=0, thecomparator 203 will resolve the incoming signal as being a ‘1’ or ‘0’ bydetermining whether the signal is above or below the partial responselevel, −α. Because both comparisons are performed for each incomingsymbol, the selection of which comparator output represents the state ofthe symbol may be delayed until the state of the preceding symbol isresolved. In the partial response receive circuit 200, for example, thesample values output by the comparators 201 and 203 are output to thedecision circuit 204 where they are optionally stored in storageelements 208 and 209 (e.g., D flip-flops or other types of storagecircuits) and provided to respective input ports of a select circuit 205(e.g., a multiplexer). The sample value selected by the select circuit205 is stored in a storage circuit 207 in response to a sampling clocksignal 210 (or other timing control signal) at which point the samplevalue becomes the D_(N−1) sample value. The D_(N−1) sample value storedwithin storage circuit 207 is fed back to the select input of the selectcircuit 205 to select one of the two sample values generated by thecomparators 201 and 203. That is, the D_(N−1) sample value is used toselect, via select circuit 205, which of the comparators 201 and 203will source the D_(N) sample. In an embodiment that includes the storageelements 208 and 209, the output of storage circuit 207 becomes theD_(N−2) sample value and is used to select one of the D_(N−1) samplevalues output from the storage elements 208 and 209.

Reflecting on the operation of the partial response receive circuit 200of FIG. 7 in reference to the signal distribution shown in FIG. 6, itcan be seen that the signaling margin (i.e., difference between signallevels representative of ‘1’ and ‘0’ states) within either of thecomparators 201, 203 exceeds the signaling margin between the worst-casepartial response states. That is, the signaling margin between theworst-case partial response states (10-to-01) is (1−α)−(−1+α)=2−2α,while the signaling margin in either pair of the separated signalconstellation is (1−α)−(−1−α)=2. Thus, as alpha grows, the signalingmargin in a single-comparator receiver decreases, going to zero as αapproaches 1. By contrast, the signaling margin in the partial responsereceive circuit 200 of FIG. 7 remains substantially constant at 2, evenas α grows. In effect, by offsetting the threshold of the comparators201 and 203 by the partial response levels +α and −α, respectively, thepartial response to the preceding symbol is canceled, making the fullresponse of the incoming symbol available to resolve the symbol state.

Still referring to FIG. 7, a threshold generator 211 is used to generatethe ±α thresholds supplied to the comparators 201 and 203. In the caseof a single-ended signaling system, the threshold values ±α may bevoltage levels applied to reference inputs of the comparators 201 and203, respectively, to enable pseudo-differential signal detection (i.e.,comparison of the incoming signal level with the threshold voltage levelto resolve the digital state of the sample value). In the case of adifferential signaling system, the threshold values ±α may be voltagesor currents applied to offset the common modes of the comparators to the±α voltage levels depicted in FIG. 6. The threshold values ±α may alsobe digital values for controlling digital-to-analog converters (DACs)within the comparators 201 and 203. In one embodiment, the thresholdgenerator 211 includes circuitry for a one-time or periodicdetermination of the ±α threshold levels, for example in one or morecalibration operations. In other embodiments, the ±α threshold levelsare adaptively generated based on incoming signal levels produced byselected data patterns. Embodiments of the threshold generator 211 arediscussed in further detail below.

Multi-Data-Rate Signaling

In the partial response receive circuit 200 of FIG. 7, a single symbolis captured during each cycle of the sampling clock signal 210. That is,a rising (or falling) edge of the sample clock is used to capturesamples of the incoming data signal. In a multi-data rate system,multiple symbols are captured per cycle of the sampling clock signal210. In such systems, clock generation circuitry is provided to generatemultiple instances of the sampling clock signal 210 that arephase-distributed through a period (1/frequency) of the sampling clocksignal. FIG. 8, for example, depicts a double data rate signalingprotocol in which two symbols are transmitted in succession during eachperiod (i.e., cycle time) of a sampling clock signal. Accordingly, twoinstances of the sampling clock signal are provided: an even-phasesampling clock signal, SCLK_(E), to sample even-numbered symbols D_(N),D_(N+2), D_(N+4) . . . ; and an odd-phase sampling clock signal,SCLK_(O), to sample odd-numbered symbols D_(N−1), D_(N+1), D_(N+3) . . .. This technique may be extended to achieve virtually any data rate,including quad data rate (4 symbols per sampling clock cycle), octaldata rate (8 symbols per sampling clock cycle), decade data rate (10symbols per sampling clock cycle), and so forth.

FIG. 9 illustrates a partial response receive circuit 215 for use in adouble data rate signaling system. The partial response receive circuit215 includes an odd-phase receive circuit 216 clocked by an odd-phasesampling clock signal 210 _(O) (SCLK_(O)), and an even-phase receivecircuit 217 clocked by an even-phase sampling clock signal 210 _(E)(SCLK_(E)). The odd- and even-phase receive circuits 216, 217 aresimilar to the partial response receive circuit 200 of FIG. 7, exceptthat, due to the interleaved nature of the samples they generate, theselect circuit 225 of the odd-phase receive circuit 216 is controlled bya latched instance of a sample selected by the select circuit 239 in theeven-phase receive circuit 217 and, conversely, the select circuit 239of the even-phase receive circuit 217 is controlled by a latchedinstance of a sample selected by the select circuit 225 in the odd-phasereceive circuit 216. Samples D_(N) ⁺/D_(N) ⁻are generated by comparators232 and 233, stored in storage circuits 235 and 237, and then selectedby select circuit 239 to form the even-phase pipeline (EVEN PIPE)illustrated in FIG. 10 (the output of the select circuit 239 optionallybeing buffered in storage circuit 243 in response to the even-phasesampling clock signal 210 _(E)). Similarly, samples D_(N+1) ^(+/)D_(N+1)⁻ are generated by comparators 218 and 219, stored in storage circuits221 and 223, then selected by the select circuit 225 form the odd-phasepipeline (ODD PIPE) illustrated in FIG. 10 (the output of the selectcircuit 225 optionally being buffered in storage circuit 227 in responseto the odd-phase sampling clock signal 210 _(O)). Thus, when an oddphase sample D_(N−1) is selected by select circuit 225, the D_(N−1)sample is latched within latch element 241 of the even-phase receivecircuit (thereby making D_(N−1) available for a full cycle of theeven-phase clock signal 210 _(E)) and thereafter used to select thesubsequent even-phase sample D_(N). The selected even phase sample D_(N)is then latched within latch element 228 of the odd-phase receiver andthereafter used to select the subsequent odd-phase sample, D_(N+1). Forhigher data rates, the number of partial-cycle receive circuits(circuits 216 and 217 each being a half-cycle receive circuit) may beincreased according to the data rate. For example, in a quad data ratesystem, a partial-response receiver includes four quarter-cycle receivecircuits interconnected such that sample N within a first quarter-phasereceive circuit is used to select sample N+1 within a secondquarter-phase receive circuit; sample N+1 within the secondquarter-phase receive circuit is used to select sample N+2 in a thirdquarter-phase receive circuit; sample N+2 is used to select sample N+3in a fourth quarter-phase receive circuit; sample N+3 is used to selectsample N+4 in the first quarter phase receiver; and so forth. In theremainder of this description, various partial response receive circuitembodiments are described in the context of a single data rate (SDR)signaling system. Each of the embodiments disclosed may be modified asdescribed in reference to FIGS. 8-10 to support multi-data ratesignaling.

Threshold Level Generation

In one embodiment of the invention, a technique referred to herein asembedded scoping is used to determine the ±α threshold levels appliedwithin the comparators of the partial response receive circuit. Embeddedscoping involves iteratively receiving a sequence of symbols in areceiver and comparing the received symbol sequence with a localgeneration of the sequence to confirm error-free reception. With eachreceive-and-confirm iteration, a threshold voltage used to distinguishbetween symbol values in the incoming signal is offset from a calibratedlevel by a progressively larger amount until a symbol in the sequence nolonger matches the expected value. The threshold voltage offset at whichthe failure occurs is referred to herein as a pass/fail offset andrepresents a measure of the signal level at the sampling instant atwhich the failure occurred. Thus, by sweeping the threshold voltagethrough a range of threshold voltages until the pass/fail offsets foreach symbol in the symbol sequence have been detected, a sample plot forthe incoming signal may be developed. Further, by sweeping the receiveclock signal through an incremental sequence of phase offsets, anddetermining the pass/fail offset at each phase offset, a complete traceof the incoming signal may be generated. Also, the granularity and startstop points of the phase offsets and/or threshold voltage steps may becontrolled (e.g., by configuring a programmable circuit or register) toenable the waveform trace to be constrained to selected points ofinterest in the incoming signal (e.g., ±N° from an intended samplinginstant, N representing a sweep angle).

FIG. 11 illustrates the use of embedded scoping to generate a time-basedtrace 290 of an incoming data signal 286. The range of threshold voltageoffsets over which the incoming signal 286 is sampled is indicated byVT, and the range of phase offsets at which the signal is sampled isindicated by φ. Each sample point within the sweep is indicated by arespective dot within a grid of sample points 280. Note that the sweepmay be obtained by stepping the voltage threshold through the range ofV_(T) values for each value of φ, or, alternatively, by stepping theclock phase through the range of φ values for each value of V_(T).

Still referring to FIG. 11, reference numeral 288 indicates a pair ofsamples for which a pass/fail condition is detected. A correspondingpass/fail offset (PFO) is determined according to the difference betweenthe calibrated V_(T) level (V_(T) (CAL)) and the average of the V_(T)offsets between the pass and fail samples, and recorded as a measure ofthe incoming signal. That is, the pass/fail offset may be used toestablish a data point within the trace 290 as shown. After sweepingthrough all the sample points within the grid 280 (which sweep may berepeated numerous times to obtain an average and to discard statisticaloutliers), a measure of the incoming signal is obtained as illustratedgraphically by the trace 290.

Embedded scoping has a number of benefits over traditional signalmeasurement techniques. First, because the technique is non-invasive(i.e., no probe contact), the electrical characteristics of the systemunder test are unaltered, thereby yielding potentially more accurateresults. Also, the trace is generated from the perspective of thereceive circuit itself, meaning that any non-ideal characteristics ofthe receive circuit are accounted for in the resulting signal traceinformation. Finally, because all components needed for embedded scopingmay be included within a finished signaling system, embedded scoping maybe used to perform numerous run-time analyses in addition to partialresponse measurement including, without limitation, determining thelatency and amplitude of reflections and other distortions within thesignaling system.

FIG. 12 illustrates a signaling system 300 according to an embodiment ofthe invention. The signaling system 300 includes a transmit device 301and receive device 309 that employ embedded scoping to determine partialresponse amplitudes. The transmit device 301 includes a patterngenerator 303, data selector 305, equalizing transmitter 307 andapplication logic 302. The application logic 302 performs the corefunction of the transmitting device (e.g., signal processing,instruction processing, routing control, or any other function) andprovides transmit data (TX DATA) to a first input of the data selector305. During normal operation, the application logic 302 outputs a logiclow scope signal 306 (SCOPE) to the data selector 305 to select thetransmit data to be passed to the equalizing transmitter 307 fortransmission to the receive device 309 via signal path 122 (which mayinclude or be connected to numerous sources of discontinuity such asconnectors, vias, stubs, etc.). During a scoping mode of operation, theapplication logic 302 drives the scope signal 306 high to enable ascoping mode of operation within the transmit device 301. In the scopingmode, the data selector 305 selects a repeating single-symbol pulsesequence (e.g., a test signal such as: 00100 . . . 00100 . . . 00100 . .. ) generated by the pattern generator 303 to be transmitted to thereceive device 309. The receive device 309 includes a partial responsereceiver 310 to receive the incoming data signal, a pattern register 311to store a local version of the single-symbol pulse sequence, amultiplexer 312 to enable the pattern register 311 to be switchedbetween load and barrel-shifting modes, a XOR gate 313 to compare thereceived data sequence with the locally generated sequence, andapplication logic 315 (or other logic) to generate a threshold voltageadjust signal (THRESH ADJ) to step the threshold voltage used within thepartial response receive circuit through their scoping ranges. In onembodiment, the thresholds applied to the multiple comparators of thepartial response receive circuit are set to the same nominal startingvalue and stepped together for purposes of embedded scoping. In analternative embodiment, only one comparator of the partial responsereceive circuit is used when scoping mode is enabled. The applicationlogic may additionally generate a clock adjust signal (not shown) tostep the sampling clock through a sequence of phase offsets within acycle of the sampling clock signal. The application logic 315additionally builds a trace record (i.e., data indicative of theincoming data sequence) based on the output of XOR gate 313.

When the receive device 309 is in a scoping mode of operation, themultiplexer 312 is initially set to load the pattern register 311 withthe output of the partial response receiver 310. After a desiredsequence of data (e.g., the single-symbol pulse sequence) is shiftedinto the pattern register 311, the multiplexer 312 is set to enable thebarrel-shifting mode of the pattern register 311. That is, themultiplexer 312 selects the output of the pattern register 311 to be fedback to the input of the pattern register 311 so that the contents ofthe pattern register 311 are continuously rotated through the patternregister 311 (i.e., a barrel shifting operation). By this arrangement,the data sequence loaded into the pattern register 311 is repeatedlyoutput, bit by bit, to a first input of the XOR gate 313. The datasequence received by the partial response receiver 310 is input to asecond input of the XOR gate 313 so that the received data sequence iscompared, bit by bit, with the data sequence stored within the patternregister 311. By selecting the length of the repeatedly transmitted datasequence to match the storage size of the pattern register 311, thepattern register contents are repeatedly compared with a newly receivedversion of the same data sequence (i.e., putatively the same datasequence). Any reception error will result in a mismatch between thereceived value and the corresponding value within the pattern registerand therefore, when compared by XOR gate 313, will result in an errorsignal being output from the XOR gate 313 to the application logic 315.The application logic 315 may then record the adjusted threshold voltage(and optionally the clock phase offset) at which the error occurred as asignal level within the waveform trace.

FIG. 13 illustrates a sample waveform trace 320 of a pulse responsecaptured by an embedded scope within the signaling system of FIG. 12. Asshown, the waveform starts and ends at a steady-state low level whichcorresponds to the −1−α level discussed in reference to FIGS. 5 and 6. Apulse (i.e., D=1) is received at time T3, and the partial response ofthe pulse is received at time T4. Due to the preceding zero-valuedsymbol, the signal level sampled at time T3 corresponds to the 1−αlevel. Similarly, due to the preceding one-valued symbol, the signallevel sampled at time T4 corresponds to the −1+α level. The differencebetween the −1+α level and the −1−α level may be determined by theapplication logic 315 of FIG. 12 (or other circuitry) and used todetermine ±α. That is, α=((−1+α)−(−1−α))/2. The normalized signal level,1, may be used in certain clock recovery operations (discussed below)and may be determined from the pulse level and the steady-state lowlevel (i.e., 1=((1−α)−(−1−α))/2). Once determined, the a level may beapplied to the comparators of the partial response receive circuit toenable partial response operation.

FIG. 14 illustrates a partial response receiver 325 that includescircuitry for adaptively generating the partial response thresholdlevels ±α. The partial response receiver 325 includes a partial responsereceive circuit 200, a level sampler 327, and an adaptive module 329.The partial response receive circuit 200 operates generally as describedabove in reference to FIG. 7 to generate a pair of sample values basedon comparisons (in comparators 201 and 203) of the incoming signal D_(N)with offset thresholds ±α. The samples are stored in storage elements208 and 209 as samples D_(N−1) ⁺ and D_(N−1) ⁻ (the ‘^(+’) and ‘^(−’)designating the samples as corresponding to relatively positive andnegative partial responses). A select circuit 205 selects one of theD_(N−1) ⁺ and D_(N−1) ⁻ samples to be the selected D_(N−1) sample basedon the state of the D_(N−2) sample stored in storage circuit 207. Asdiscussed above, in a multi-data rate system, the D_(N−2) sample may besupplied by a counterpart partial response receive circuit clocked by aphase shifted version of the sampling clock signal 210. In either case,single- or multi-data rate, two or more time-adjacent sample values aresupplied to (or buffered within) the adaptive module 329. In theembodiment of FIG. 14, sample values D_(N−2) and D_(N−1) are supplied tothe adaptive module 329, though higher or lower latency samples may beprovided in alternative embodiments.

The level sampler 327 includes four comparators 331 ₁-331 ₄ forcomparing the signal level of the incoming symbol, D_(N), with each ofthe four signal levels that correspond to the bimodal signaldistribution of FIG. 5. That is, comparator 331 ₄ compares the incomingsignal with a threshold level at 1±α and generates a corresponding errorsample E₁₁ according to whether the incoming signal is above or belowthe 1+α level. Similarly, comparators 331 ₃, 331 ₂ and 331 ₁ compare theD_(N) signal level with threshold levels at 1−α, −1+α and −1−α,respectively and generate corresponding error samples E₁₀, E₀₁ and E₀₀.Each of the error samples is buffered in a respective storage circuit335 ₁-335 ₄ before being provided to the adaptive module 329. By thisarrangement, the error samples arrive at the adaptive module 329 withthe same latency as the D_(N−1) sample generated by the partial responsereceive circuit 200.

The adaptive module 329 responds to the data samples from the partialresponse receive circuit 200 and the error samples from the levelsampler 327 by selectively updating the threshold values supplied to thecomparators 201 and 203 within the partial response receive circuit 200and comparators 331 ₁-331 ₄ within the level sampler 327. For example,when the data samples indicate a 11 state (i.e., D[N-1:N-2]=11), theadaptive module 329 increases or decreases the 1−α threshold levelaccording to whether the incoming signal level is indicated by errorsample E₁₁ to be above or below the 1±α threshold level. The adaptivemodule similarly updates the 1−α, −1+α, and −1−α thresholds based onerror samples E₁₀, E₀₁ and E₀₀ when the data samples indicate the 10, 01and 00 states, respectively. As discussed below, the ±α levels may bederived from the 1+α, 1−α, −1+α and −1−α threshold levels, or a subsetthereof.

FIG. 15 illustrates an embodiment of the adaptive module 329 of FIG. 14.The adaptive module includes a 2:4 decoder 353, up/down counters 357₁-357 ₄, averaging circuits 361 and 365, digital-to-analog converters(DACs) 359 ₁-359 ₆, and, optionally, a filter circuit 351. The 2:4decoder 353 activates one of four enable signal lines 354 ₁-354 ₄ (i.e.,asserts an enable signal on the signal line) according to the state ofthe input data samples D_(N−1) and D_(N−2) (also expressed herein asD[N−1:N−2]). Each of the counters 357 has an up/down input (U/D) coupledto receive a respective one of the four error samples (i.e., E₀₀, E₀₁,E₁₀ and E₁₁), a count enable input (CEN) coupled to a respective one ofthe enable signal lines 354, and a strobe input coupled to receive thesample clock signal 210. By this arrangement, during each cycle of thesample clock signal 210, the counter 357 coupled to the activated enablesignal line 354 is incremented or decremented according to the state ofthe corresponding error sample. Thus, when D[N−2:N−1]=11, counter 357 ₄is incremented if error sample E₁₁ indicates that the incoming signallevel is above the 1+α threshold level, and decremented if E₁₁ indicatesthat the incoming signal level is below the 1+α threshold level. Thecount values maintained within counters 357 ₃, 357 ₂ and 357 ₁ aresimilarly incremented and decremented according to error samples E10,E01 and E00, respectively, when enabled by their respective enablesignal lines 354. In one embodiment, the filter circuit 351 is used tofilter the incoming error samples, for example, by requiring apredetermined number of same-state error samples to be received within agiven time before the corresponding count value is adjusted. Other typesof error sample filtering may be applied in alternative embodiments.Also, the filter 351 may be omitted altogether.

The count values maintained within the counters 357 ₁-357 ₄ are outputas control values to respective DACs 359 ₁-359 ₄ to generate the 1+α,1−α, −1+α and −1−α levels. Thus, the adaptive module 329 operates toselectively update the 1+α, 1−α, −1+α and −1−α threshold levelsaccording to the state of the received data values D[N−1:N−2].

The averaging circuits 361 and 365 generate the ±α threshold levels byaveraging the control values output from counters 357 ₁-357 ₄. Averagingcircuit 365, for example, includes a summing circuit 366 to sum theC_(1+α) and C_(−1+α) control values from counters 357 ₄ and 357 ₂,respectively, and a divide-by-2 element 367 (which may be achievedwithout active circuitry by dropping the least significant bit of thesum) to generate a control value C_(α) that corresponds to the desired αthreshold level (i.e., (C_(1+α)+C_(−+α))/2=C_(α)). Averaging circuit 361similarly includes a summing circuit 362 to sum the count values C_(1−α)and C_(−1−α) from counters 357 ₃ and 357 ₁, and a divide-by-2 element363 to generate control value C_(−α). In one embodiment, each of thecontrol values generated by the counters 357 and the averaging circuits361, 365 are input to respective DACs 359 ₁-359 ₆ to generate thethreshold levels used within the comparators of the partial responsereceive circuit 200 and level sampler 327 of FIG. 14. In an alternativeembodiment, each of the comparators within the partial response receivecircuit 200 and level sampler 327 include internal DACs and receiverespective threshold values in the form of the control values themselves(i.e., C_(α), C_(−α), C_(1+α), C_(1−α), C_(−1+α), C_(−1−α)). Also, in adifferential embodiment (and in certain single-ended embodiments), theC_(+α) and C_(−α), values may be complements of one another so that oneof the averaging circuits 361 or 365 may be replaced by a multi-bitinverter. Further, the C_(1+α) and C_(−1−α) values may be complements,and the C_(1−α) and C_(−1+α) values may be complements such that two ofthe comparators within the level sampler 327 of FIG. 14 and two of thecounters 357 within the adaptive module 329 (and two of the DACs 359, ifused) may be omitted.

FIG. 16 illustrates an alternative circuit arrangement 375 that may beused in place of the averaging circuits 361 and 365 of FIG. 15. Insteadof averaging the C_(1+α) and C_(−1+α) count values to generate theC_(+α) value, C_(+α) is generated by halving the difference between theC_(1+α) and the C_(1−α) control values (i.e.,C_(α)=((C_(1+α))−(C_(1−α)))/2). Similarly, the C_(−α) value is generatedby halving the difference between the C_(−1−α) and the C_(−1+α) controlvalues. Thus, the circuit of FIG. 16 includes a difference circuit 376to generate 2α by subtracting the C_(1−α) control value from the C_(1+α)control value, and a divide-by-2 element 377 (which may be a implementedby dropping the least significant bit of the difference) to generateC_(α) by halving the 2α value. Difference circuit 379 and divide-by-2element 380 are used in a similar manner to generate C_(−α) from countvalues C_(−1+α) and C_(−1−α) in a corresponding manner.

Differential Comparator

FIG. 17 illustrates a comparator 400 that may be used within thereceiver circuits and level sampling circuits of FIGS. 7, 9 and 14. Thecomparator 400 includes a preamplifier 401 and a sampling circuit 425.The preamplifier 401 includes a pair of differential amplifiers 402 and403 each biased by a respective current DAC (IDAC) 411 and 413, and eachhaving first and second output nodes 418 and 419 coupled to a supplyvoltage via a respective resistive element, R. The resistive elementsmay be implemented, for example, using diode-configured transistors,biased transistors, resistors, or any other active or passive circuitryfor establishing a resistance. Transistors 405 and 404 withindifferential amplifier 402 have widths W1 and W2, respectively, with W1being greater than W2. Transistors 408 and 407 within differentialamplifier 403 also have respective widths W1 and W2. A differentialinput signal composed of signal components D and /D is coupled to eachof the differential amplifiers 402, 403 such that D is coupled totransistors 404 and 408 and /D is coupled to transistors 405 and 407. Bythis arrangement, when C_(α), and /C_(α), are substantially equal (e.g.,in an 8-bit control word, C_(α)=128 and /C_(α)=127), the differentialamplifiers 402 and 403 are substantially balanced, operating in effectas a single differential amplifier having component transistors of widthW1+W2. Thus, if D is greater than /D, transistors 404 and 408 willcollectively sink more current than transistors 405 and 407, therebycausing the voltage on output node 418 to be pulled down (i.e., via theresistive element, R, coupled to the output node 418) more than thevoltage on output node 419.

When the preamplifier 401 is balanced (i.e., C_(α) substantially equalto /C_(α)), the voltages on the preamplifier output nodes 418 and 419are substantially equal when D and /D are at the common mode potential(i.e., as when D and /D cross one another in transition). Thus, theeffective threshold of the preamplifier 401, and therefore thecomparator 400 as a whole, occurs at the common mode of D and /D. Bycontrast, when the preamplifier is imbalanced by increasing C_(α)relative to /C_(α), equal values of D and /D result in output node 419being pulled lower than output node 418, due to the fact that transistor405 is wider than transistor 404 (and therefore has a greater gain), andthat the compensating (balancing) effect of differential amplifier 403is diminished by the reduced control value /C_(α). Thus, increasingC_(α) relative to /C_(α) increases the effective threshold of thepreamplifier above the common mode. By increasing C_(α) to the point atwhich the threshold between ‘0’ and ‘1’ signal levels is set to a, adifferential comparator having a threshold level at a is achieved. Byreversing the connections of the C_(α) and /C_(α) values to the currentDACs of a counterpart comparator (not shown), a differential comparatorhaving a threshold level at −α is achieved.

The sampling circuit 425 includes a differential amplifier 426 formed bytransistors 423 and 424, a sense amplifier 427 formed by back-to-backcoupled inverters 428 and 429, and a storage circuit 436 formed by aset-reset flip-flop. The differential amplifier 426 includes controlinputs coupled to the output nodes 418 and 419, respectively, of thepreamplifier 401, and output nodes 431 and 433 coupled to sourceterminals of the inverters 428 and 429, respectively. A biasingtransistor 430, switchably controlled by the sampling clock signal 210(or other sample control signal), is coupled between the differentialamplifier 426 and a ground reference (or other low voltage reference).The sampling clock signal 210 is additionally coupled to control inputsof positively-doped MOS (PMOS) transistors 434 and 435 which are coupledbetween a supply voltage (e.g., V_(DD)) and output nodes of theinverters 428 and 429. By this arrangement, when the sample clock signal210 is low, transistor 430 is switched off, and transistors 434 and 435are switched on to pre-charge the output nodes of the inverters 428 and429 to the supply voltage. The output nodes of the inverters 428 and 429are coupled to active-low set and reset inputs, respectively, of thestorage circuit 436, so that the content of the storage circuit 436 ismaintained through the low half-cycle of the sample clock signal 210.When the sample clock signal 210 goes high, biasing transistor 430 isswitched on and draws current through the two transistors 424 and 423 ofthe differential amplifier 426 in proportion to the voltages developedon the output nodes 418 and 419 of the preamplifier 401. Thus, if thevoltage developed on node 419 is higher than the voltage on node 418,the current drawn by biasing transistor 430 will flow primarily throughtransistor 423. Conversely, if the voltage developed on node 418 ishigher than the voltage on 419, the current drawn by biasing transistorwill flow primarily through transistor 423. Transistors 434 and 435 areswitched off in response to the high-going sample clock signal 210, sothat the pre-charged outputs of the inverters 428 and 429 are dischargedby currents flowing through transistors 423 and 424. By this operation,if the incoming signal (D) exceeds the common mode voltage, ((D+/D)divided by 2), by more than the +α threshold level (i.e., the incomingsignal exceeds the +α threshold level), the current drawn by biasingtransistor 430 will flow primarily through transistor 423. Consequently,the output node of inverter 429 will be discharged more rapidly than theoutput node of inverter 428, driving the output of inverter 429 low anddriving the output of inverter 428 high (i.e., the PMOS transistorwithin inverter 428 is switched on and the NMOS transistor withininverter 428 is switched off). The low output of inverter 429 is appliedto the active-low set input of the storage circuit 436, causing thestorage circuit 436 to store a logic ‘1’ sampled data value. Bycontrast, if the incoming signal level does not exceed the +α thresholdlevel, the current drawn by biasing transistor 430 will flow primarilythrough transistor 424, thereby driving inverter 428 low (and drivinginverter 429 high) to store a logic ‘0’ sampled data value withinstorage circuit 436.

FIG. 18 illustrates an embodiment of the current DAC 411 of FIG. 17. Thecurrent DAC 411 includes control transistors 437 ₀-437 _(N−1) andbiasing transistors 439 ₀-439 _(N−1) Each of the control transistors 437₀-437 _(N−1) is coupled in series (e.g., source to drain) with acorresponding one of the biasing transistors 439 ₀-439 _(N−1) to form atransistor pair that is coupled between a reference voltage (ground inthis example) and an output node 438 (i.e., the node to be connected tothe source terminals of the transistors which form the differentialamplifier 402). Gate terminals of the control transistors 437 ₀-437_(N−1) are coupled to receive respective component signals,C_(α)[0]-C_(α)[N−1], of a multi-bit control value, C_(α) (or, controlvalue /C_(α)). Each of the control transistors 437 ₀-437 _(N−1) has abinary weighted gain such that a current of I_(REFZ) x2 ^(i) (where irepresents the i^(th) transistor in the positions 0, 1, 2, . . . , N−1)flows through control transistor 437 _(i) when the corresponding controlsignal component is high. Thus, if all the constituent bits of thecontrol value C_(α)[N−1:0] are high, then I_(REF) flows through controltransistor 437 ₀, I_(REF) x2 flows through transistor 437 ₁, I_(REF) x4flows through control transistor 437 ₂, and so forth to controltransistor 437 _(N−1) which conducts I_(REF) x2 ^(N−1). Accordingly,control transistors 437 ₀-437 _(N−1) are designated x1, x2 . . . , x2^(N−1) transistors, respectively. By this arrangement, the control valueC_(α) [N−1:0] may be set to any of 2^(N) values to select bias currentsthat range from 0 to I_(REF) x2 ^(N−1) i increments of I_(REF). Thebiasing transistors 439 ₀-439 _(N−1) have gate terminals coupled toreceive a bias voltage, VBIAS, that is adjusted as necessary (e.g., by abiasing circuit) to establish or maintain a desired I_(REF).

In one embodiment, the relative gains (i.e., transconductance values) ofthe various transistors used to implement the current DAC 411 (andtherefore drive strengths of the sub-drivers) are established byadjusting the width-length ratio (i.e., W/L) of individual controltransistors 437 and/or biasing transistors 439. For example, thewidth-length ratio of the x2 control transistor 437 ₁ is twice thewidth-length ratio of the x1 control transistor 437 ₀, the width-lengthratio of the x4 control transistor 437 ₂ is twice the width-length ratioof the x2 control transistor 437 ₁, and so forth. The biasingtransistors 439 may have similar gain ratios relative to one another(e.g., x1, x2, x4, x2 ^(N−1) as shown in FIG. 18). Other techniques foradjusting the relative gains of the control transistors 437 and biasingtransistors 439 may be used in alternative embodiments. Also, weightingsother than binary weightings may be used. For example, in oneembodiment, each of the control transistors 437 has an equal gain toeach of the other control transistors 437 such that the current drawn bythe current DAC 411 is proportional to the number of logic ‘1’ bits inthe control value, C_(α).

FIG. 19 illustrates an alternative embodiment of a differentialcomparator 450 that may be used to implement the comparators depicted inFIGS. 7, 9 and 14. The comparator 450 includes a sampling circuit 452and an offset control circuit 440. The sampling circuit 452 isimplemented in generally the same manner as the sampling circuit 425 ofFIG. 17 (and includes differential amplifier 426, sense amplifier 427,biasing transistor 430, and storage circuit 436), except that the inputsignal lines carrying D and /D are coupled directly to the controlterminals of transistors 423 and 424, respectively. The offset controlcircuit 440 includes a differential amplifier 446 having output nodescoupled to nodes 431 and 433 of the sampling circuit 452. Controlterminals of the transistors 447 and 449 of the differential amplifier446 are biased by respective voltage DACs (e.g., implemented by currentDACs 443 and 445 coupled to respective resistive pull-up elements 451and 453) controlled by C_(α) and /C_(α), respectively. By thisarrangement, when the sample clock signal goes high the current throughoutput node 433 of the sampling circuit 452 is a sum of the currentsdrawn by transistor 423 of the sampling circuit 452 and transistor 447of the offset control circuit 440. Similarly, the current through node431 of the sampling circuit 452 is a sum of the currents drawn bytransistor 424 of the sampling circuit 452 and transistor 449 of theoffset control circuit 440. As discussed above in reference to FIG. 17,when the current through node 431 exceeds the current through node 433,a logic ‘1’ is stored within storage circuit 436 and, conversely, whenthe current through node 433 exceeds the current through node 431, alogic ‘0’ is stored within storage circuit 436.

When the DAC control values C_(α) and /C_(α) are substantially the same,the comparator 450 is balanced and the effective threshold occurs at thecommon mode of the D and /D signal levels. That is, if D exceeds thecommon mode voltage, V_(CM)=((D+/D) divided by 2), the current throughnode 433 exceeds the current through node 431, causing a logic ‘1’ to becaptured as the sampled data value. As C_(α) is increased and /C_(α)decreased, the effective threshold of the differential amplifier isincreased such that D must be higher than /D by an amount necessary toovercome the additional current drawn by transistor 449 of the offsetcontrol circuit. Thus, by increasing C_(α) and decreasing /C_(α), theeffective threshold of the sampling circuit may be set to the partialresponse level, α. That is, a logic ‘1’ is output as the sampled datavalue if the difference between the D exceeds the common mode voltage,V_(CM) by more than α, and a logic ‘0’ is output otherwise. Acounterpart comparator having a −α threshold may be provided byswitching the C_(α) and /C_(α) inputs to the DACs 443 and 445.

Threshold Level Generator with Single Level-Adapting Comparator

FIG. 20 illustrates a partial response receiver 500 according to anotheralternative embodiment. The partial response receiver 500 includes apartial response receive circuit 200 that operates generally asdescribed in reference to FIGS. 7 and 14, a level sampler 501, and anadaptive module 503. In contrast to the level sampler 327 of FIG. 14,the level sampler 501 includes a single comparator 507 that generates anerror sample, E_(DLEV), according to whether the incoming signal exceedsor is below a data level threshold generated by the adaptive module 503.In the embodiment of FIG. 20, the data level threshold is established bya multi-bit control value, C_(DLEV), generated by the adaptive module.In alternative embodiments, the data level threshold may be establishedby an analog voltage or current generated by the adaptive module 503.The error sample may be buffered in one or more storage circuits 509before being forwarded to the adaptive module 503, thereby aligning thelatency of the error sample with data samples generated by the partialresponse receive circuit 200.

The adaptive module 503 receives the error sample from the level sampler501 and data samples D_(N−1) and D_(N−2) from the partial responsereceive circuit 200 and, in response, generates control values, C_(α)and /C_(α), to establish the ±α levels within the partial responsereceive circuit 200, and the control value, C_(DLEV), to establish thedata level threshold within the level sampler 501. In one embodiment,the adaptive module 503 initially updates the C_(DLEV) value upondetecting reception of the 1+α data pattern (i.e., D[N−1:N−2]=11) untilthe comparator 507 indicates that the data level threshold matches the1+α data level. Thereafter, the adaptive module 503 updates the C_(DLEV)value in response to the 1−α data pattern (10) until the D_(LEV)threshold matches the 1−α data level. The control value, Cα, may begenerated, for example, by halving the difference between the twoC_(DLEV) values (i.e., C_(1+α) and C_(1−α)), and the /Cα control valuemay be generated by complementing the C_(α) value.

FIG. 21 illustrates the operation of the adaptive module 503 of FIG. 20according one embodiment. Initially, at block 521, the control values,C_(α) and C_(DLEV), and an internally maintained control value, C_(2α),are each set to a nominal zero value. In one embodiment, the nominalzero value corresponds to a DAC midpoint value. For example, in an 8-bitDAC, a midpoint value of 1000 0000b (the ‘b’ indicating binary notation)or 128 decimal may be used as the nominal zero value. The C_(−α) valuemay be generated by subtracting the C_(α) value from the full-scale DACvalue. Thus, in the eight-bit example, when C_(+α)=128,C_(−α)=(2⁸−1)−128=127. It will be appreciated that this result may beobtained by complementing C_(α) (i.e., /C_(α)=C_(−α)).

At decision block 523, a history of increment and decrement operationsapplied to C_(DLEV) value is evaluated to determine whether thethreshold level that corresponds to the 1+α signal level has beenreached (i.e., UPPER DLEV FOUND). In one embodiment, if, over the last Nupdates to the C_(DLEV) value, the difference between the number ofincrement operations and the number of decrement operations is less thana predetermined value, the data level threshold is deemed to bedithering about the 1+α signal level and the upper data level thresholdis therefore considered found. If the upper data level threshold has notbeen found, then the sample values generated by the partial responsereceive circuit (D[N−1:N−2]) are evaluated in decision block 525 todetermine whether a signal level at the 1+α level has been received. Ifso, the error sample generated by the level sampler, C_(DLEV), isevaluated in decision block 527 to determine whether the incoming signallevel is greater than or less than the threshold level established bythe present value of C_(DLEV). If the error sample is a ‘1’, theincoming signal level is greater than the threshold level, and theC_(DLEV) value is incremented at 529 to increase the data levelthreshold. If the error sample is a ‘0’, the signal level is less thanthe data level threshold and the C_(DLEV) value is decremented at 531 todecrease the data level threshold. As discussed above in reference toFIG. 15, some level of filtering may be applied before incrementing ordecrementing the C_(DLEV) value. After the C_(DLEV) value has beenincremented or decremented (i.e., in block 529 or 531), the C_(DLEV)increment/decrement history is evaluated again at decision block 523 todetermine whether the upper data level threshold has been found.

Returning to decision block 525, if D[N−1:N−2] is not equal to ‘11’, theerror sample, C_(DLEV), is not evaluated and the operation at 525 isrepeated for a subsequent set of data samples. Thus, in thelevel-adapting loop formed by blocks 523-531, the adaptive moduleselectively updates the C_(DLEV) value, and therefore the data levelthreshold according to the state of the data samples generated by thepartial response receive circuit (i.e., updates the C_(DLEV) value inresponse to detection of the 1+α sample pattern).

If, at decision block 523, the upper data level is deemed to be found,the history of increment and decrement operations is cleared and asecond level adapting loop is begun at decision block 535. At decisionblock 535, the history of increment and decrement operations isevaluated to determine whether a data level threshold corresponding tothe 1−α data level has been found (i.e., LOWER DLEV FOUND). In oneembodiment, if, over the last N updates to the C_(DLEV) value, thedifference between the number of increment operations and the number ofdecrement operations is less than a predetermined value, the data levelthreshold is deemed to be dithering about the 1−α signal level and thelower data level threshold is therefore considered found. If the lowerdata level threshold has not been found, then the sample valuesgenerated by the partial response receive circuit, D[N−1:N−2], areevaluated in decision block 537 to determine whether a signal level atthe 1−α level has been received. If not, decision block 537 isrepeatedly entered for subsequent sample value pairs until a signallevel at the 1−α level has been received. When data samples thatcorrespond to the 1−α level are detected, the error sample generated bythe level sampling circuit, C_(DLEV), is evaluated at decision block 539to determine whether the incoming signal level is above or below thedata threshold level established by the present value of C_(DLEV). Ifthe error sample is a ‘0’, as it will be initially, the signal level isbelow the data threshold level, and the C_(DLEV) value is decremented at543 to decrease the DLEV threshold level, and the C_(2α) control valueis incremented to track the difference between the upper and lower datalevel thresholds. If the error sample is determined to be a ‘1’ atdecision block 537, then C_(DLEV) is incremented and C_(2α) decrementedat block 541. By this arrangement, when the data level threshold reachesthe 1−α data level, the control value, C_(2α), will be representative oftwice the +α threshold level. Accordingly, at block 545, the C_(α) valueis generated by dividing C_(2α) by 2 (e.g., by right shifting orotherwise dropping the least significant bit of C_(2α)). The C_(−α)value may be generated by complementing the C_(α) value. Thereafter, theprocess is repeated starting at decision block 523. Alternatively, anupdated C_(2α) value may be generated by counting the return incrementsfrom the lower data level threshold to the upper data level threshold,and another updated value of C_(2α) generated on a subsequent return tothe lower data level threshold, and so forth. The C_(α) value may beupdated after each regeneration of the C_(2α) value. Also, rather thanfinding the 1+α and 1−α data levels, the 1+α and −1+α data levels may bedetermined, and the corresponding data level control values averaged togenerate the C_(α) value.

Dual Mode Receiver

FIG. 22 illustrates an embodiment of a dual mode receiver 550 that maybe operated in either a multi-level signaling mode or a partial responsemode. When a mode select signal 572 (MSEL) is in a first state (e.g., alogic low state), a two-bit per symbol, multi-level signaling mode isselected. In one embodiment, illustrated in FIG. 23, the signal level ofeach incoming symbol falls into one of four voltage ranges distinguishedby three threshold levels. The four voltage ranges are designated 10,11, 01 and 00 in FIG. 23 according to the pair of data bits representedat each level. Because each symbol constitutes a pulse having one offour possible amplitudes, the incoming multi-level signal is referred toherein as a 4-PAM (Pulse Amplitude Modulation) signal. Different PAMlevels (e.g., 8-PAM, 10-PAM, etc.) may be used in alternativeembodiments.

In the multi-level signaling mode, comparator 557 compares the incomingsignal with a midlevel threshold (referred to herein as the zerothreshold) to determine the state of the most significant bit (MSB) ofthe symbol. In the embodiment of FIGS. 22 and 23, the MSB is resolved tobe a ‘1’ if the signal level exceeds the zero threshold, and a ‘0’ ifthe signal level does not exceed the zero threshold. Comparators 553 and555 compare the incoming signal with threshold levels +α and −α,respectively. For example, in a 4-PAM signal mode, the +α thresholdlevel is set to the steady-state low level plus ⅚ of the differencebetween the steady-state high and low levels, and the −α threshold isset to the steady-state low level plus ⅙ of the difference between thesteady-state high and low levels. Referring to FIG. 23, if the incomingsignal level exceeds the +α threshold level, both comparators 553 and555 generate a logic high sample value, and if the signal level is belowthe −α threshold level, both comparators 553 and 555 generate a logiclow sample value. By contrast, if the incoming signal level is betweenthe +α and −α threshold levels, the comparators 553 and 555 generateoutputs having different states. Thus, by assigning the leastsignificant bit (LSB) of the incoming symbol to be a logic ‘0’ in thecase of a signal level above the +α threshold level or below the −αthreshold level, and a logic ‘1’ in the case of a signal level betweenthe +α and −α thresholds, the LSB may be generated by exclusive ORingthe sample values generated by the comparators 553 and 555. Accordingly,exclusive OR gate 561 is coupled to receive the sample values generatedby the comparators 553 and 555 (i.e., after the sample values arebuffered in storage circuits 554 and 556), and outputs the LSB to afirst port of select circuit 565. When the mode select signal selectsthe 4-PAM mode of the dual-mode receiver, the LSB is selected to bepassed to next stage storage circuits 567 and 578, and ultimately isselected by select circuit 579 to be provided to the adaptive module581. Thus, in 4-PAM mode, the adaptive module 581 receives an LSB andMSB (the MSB bit being stored in succession in storage circuits 558, 571and 573 for latency alignment with the LSB) for each set of samplevalues generated by the comparators 553, 555, and 557.

It should be noted that the threshold levels and data signal levelsdescribed in reference to FIG. 23 may be used in both differential andsingle-ended signaling systems. In the case of single-ended signalingsystems, the voltage levels of the thresholds and data signals areexpressed as an offset from a common, substantially fixed referencevoltage, such as a ground reference. In differential signaling systems,the data signals and thresholds are each represented by differentialsignal pair, with the voltage level of the data signal or thresholdbeing the voltage difference between the two differential signals (e.g.,subtracting the voltage level of one signal from the other).

Still referring to the 4-PAM mode of the dual mode receiver 550, thecomparator 551 generates an error sample C_(DLEV) that is bufferedwithin storage circuits 552 and 570, then provided to the adaptivemodule 581. In one embodiment, the adaptive module generates the 4-PAMmode threshold level control values C₀, C_(+α) and C_(−α) (i.e., thecontrol values supplied to comparators 557, 553, 555, respectively) bydetermining the data signal level at MSB/LSB=11, and then the signallevel at MSB/LSB=00. For example, the C_(+α) value is offset from the 00signal level by two-thirds of the difference between the 11 and 00signal levels, C₀ is set to the midpoint between the 11 and 00 signallevels, and C_(α) is offset from the 00 signal level by one-third of thedifference between the 11 and 00 signal levels. As with the adaptivemodule described in reference to FIG. 14, the error sample generated bythe comparator 551 may be filtered to obtain the steady state 11 signallevel and the steady state 00 signal level.

Still referring to FIG. 22, when the mode select signal 572 is high, thedual mode receiver 550 is switched to the partial response mode. In thepartial response mode, the comparators 553 and 55 are used to comparethe incoming data signal against the partial response threshold levels+α and −α, respectively. The samples values generated by the comparators553 and 555 are buffered in storage circuits 554 and 556, respectively,then provided to select circuit 563 which selects one of the samplesaccording to the state of the previously received sample. That is, oneof the D_(N−1) samples stored in storage circuits 554 and 556 isselected to be stored in storage circuit 567 according to the D_(N−2)sample previously stored in the storage circuit 567. The D_(N−2) sampleis stored in the storage circuit 578 to generate the D_(N−3) samplevalue. During a given symbol time, both the D_(N−2) and D_(N−3) samplesare provided to the adaptive module 581 via select circuits 579 and 575,respectively. Thus, in 4-PAM mode, the adaptive module 581 receives theMSB/LSB pair for each incoming 4-PAM symbol, and in partial responsemode, the adaptive module 581 receives the D[N−2:N−3] sample values thatrepresent one of the four states of the bimodal signal illustrated inFIG. 4.

Still referring to FIG. 22, in partial response mode, the comparator 551and adaptive module 581 operate to generate the +α and −α levels in themanner described in reference to FIGS. 20 and 21. Alternatively, becausethe MSB comparator 557 is not otherwise used in the partial responsemode, the comparator 557 may be used to determine the level of the 1−αpartial response state, thereby enabling both the 1+α and 1−α signallevels to be determined simultaneously. The ±α a levels may then begenerated based on the 1+α and the 1−α signal levels (i.e.,C_(α)=((C_(1+α))−(C_(1−α)))/2, and C_(−α)=/Cα). In another alternativeembodiment, the MSB comparator may be used to determine the −1+α signallevel, thereby enabling ±α to be determined by averaging the 1+α and−1+α signal levels. The error signal generated by the MSB comparator,E₁₀ (or E₀₁) is illustrated by dashed line 574 to indicate its optionalnature.

Reflecting on the dual mode receiver 550 of FIG. 22, it can be seen thatmuch of the circuitry provided for 4-PAM operation (e.g., the fourcomparators (551, 553, 555 and 557), storage circuits (552, 554, 556,558, 567, 570, 571, 578 and 573) and adaptive module 581) is re-used inthe 2-PAM partial response mode, thereby providing partial responseoperation with relatively little additional hardware overhead. The modeselect signal 572 may be provided from an external source or from aconfiguration control circuit within the integrated device that includesthe dual mode receiver 550. In one embodiment the configuration controlcircuit is a configuration register that may be programmed by one ormore other integrated circuits within a system (e.g., a host processoror similar control device) to establish the mode of the dual modereceiver 550. The configuration control circuit may also be anon-volatile control circuit such as a non-volatile memory, fusiblecircuit or similar circuit that may be programmed with a mode selectvalue. In yet other embodiments, the configuration control circuit maydynamically change the state of the mode select signal in response todetecting selected system conditions (e.g., detection of threshold errorrate when in one operating mode or the other).

Clock Recovery

FIG. 24 illustrates an embodiment of a partial response receiver 600that performs a clock data recovery (CDR) function. That is, the partialresponse receiver 600 recovers both data and clocking information fromthe incoming data signal, D_(N). The partial response receiver 600includes a data receive and level sampling circuit 601, adaptive module603, clock recovery circuit 605, edge-sampling comparator 607, and oneor more storage circuits 609. The data receive and level samplingcircuit 601 operates as described above in reference to FIGS. 14 and 20to generate one or more error samples, C_(DLEV), and data samplesD[N−1:N−2]. The data receive and level sampling circuit 601 may also beoperable in a multi-PAM mode as described in reference to FIG. 22. Theadaptive module 603 generates one or more threshold control values thatare supplied to the data receive and level sampling circuit 601,including one or more data level control values, C_(DLEV), and partialresponse control values, C_(+α) and C_(−α). The control values are usedto establish threshold values in the partial response compare circuits(i.e., the ±α comparators) and one or more level adapting comparators asdiscussed above.

The clock recovery circuit 605 generates a sampling clock signal 210(SCLK) and edge clock signal 610 (ECLK) in response to a transitionsample, T_(N−1), generated by the edge-sampling comparator 607 and thedata and error samples generated by the data receive and level samplingcircuit 601 (i.e., C_(DLEV) and D[N−1:N−2]). The sampling clock signal210 is used to time the operation of comparators and storage circuitswithin the data receive and level sampling circuit 601 and, at least inone embodiment, is phase aligned with midpoints in the incoming dataeyes (i.e., midpoint of data valid intervals in the incoming datasignal, D_(N)). In an alternative embodiment, the sampling clock signal210 may be offset from the midpoints in the incoming data eyes, forexample, to accommodate asymmetric setup and hold time requirements inthe comparators or other circuitry in the data receive and levelsampling circuit 601. Note that while only a single sampling clocksignal 210 is shown, multiple sampling clocks may be generated by theclock recovery circuit to enable receipt of multi-data rate signals. Forexample, in a double data rate system, the clock recovery circuit maygenerate SCLK and /SCLK to enable capture of data and signal levelsamples in both odd and even phases of the sampling clock signal 210.

The clock recovery circuit 605 adjusts the phase of the edge clocksignal 610 to maintain phase alignment between the edge clock signal 610and transition points between incoming data eyes (i.e., the edge clocksignal 610 is edge aligned with data valid intervals in the incomingdata signal). The edge clock signal 610 is supplied to the edge-samplingcomparator 607 where it is used to time the sampling of transitions inthe incoming data signal, and to one or more storage circuits 609provided to latency-align the transition sample, TN_(N−1), with the datasample, D_(N−1). In the case of a low-to-high data signal transition, alogic ‘1’ transition sample (i.e., TN_(N−1)=1) indicates that the edgeclock transition occurred after the transition in the data signal andtherefore that the edge clock signal 610 lags the data signaltransition. Conversely, a logic ‘0’ transition sample indicates that theedge clock transition occurred prior to the low-to-high data signaltransition and therefore that the edge clock signal 610 leads the datasignal transition. The transition samples from edge-sampling comparator607 and data samples from the data receive and level sampling circuit601 are used within the clock recovery circuit 605 to adjust the phaseof the edge clock signal 610 as necessary to maintain alignment betweenthe edge clock signal 610 and transitions in the incoming data signal.In one embodiment, the sampling clock signal 210 is maintained at asubstantially constant phase offset from the edge clock signal 610 suchthat phase alignment between the edge clock signal 610 and data signaltransitions yields a desired phase alignment between the sampling clocksignal 210 and midpoints in the incoming data eyes.

FIG. 25 illustrates a number of possible data signal transitions whenthe incoming data signal has the bimodal distribution shown in FIG. 5.That is, if the incoming data signal has one of the four bimodal signallevels corresponding to the partial response states 11, 10, 01, 00, thenthe signal may transition from either the 11 or 10 level to the 01 level(i.e., D[N−1:N−2]=11 or 10 and D[N:N−1]=01), and the signal maytransition from either the 00 or 01 level to the 10 level. Note thatsignal transitions from the 10 level to the 11 level (illustrated bydashed line 631) and from the 01 level to the 00 level (illustrated bydashed line 632) are also possible.

Considering the transitions from 10-to-01 and from 01-to-10, it can beseen that each of these transitions crosses the midpoint threshold(i.e., zero threshold level) at a time, T1, midway between the centersof the data eyes 628 and 630. That is, if the edge clock signal 610 isphase aligned to the transitions in the incoming data signal, the10-to-01 and 01-to-10 data signal transitions cross the midpointthreshold coincidentally (i.e., in phase alignment) with the edge clocktransition. Accordingly, the 10-to-01 and 01-to-10 transitions may becompared with the zero threshold level (designated ‘0’ in FIG. 25) togenerate transition samples for adjusting the phase of the edge clocksignal. Note that, because the signal swings in the transitions from11-to-01 and 00-to-10 are not symmetric with respect to the zerothreshold level, the 11-to-01 and 00-to-10 transitions do not cross thezero threshold level at the same time as the 10-to-01 and 01-to-10transitions, but rather cross the zero threshold level at some timelater (indicated by circle 636). Consequently, use of the 11-to-01 and00-to-10 transitions to determine zero-crossing times (i.e., times atwhich the zero threshold level is crossed) may introduce phase errorand/or bi-modal jitter in the recovered edge and sampling clock signals610 and 210. Accordingly, in one embodiment of the invention,transitions in the incoming data signal are selected according to theirzero-crossing characteristics, with the selected transitions being usedto adjust the phase of the edge and sampling clock signals 610 and 210.

FIG. 26 illustrates an embodiment of a clock recovery circuit 650 thatadjusts the phase of edge clock signal 610 and sampling clock signal 210based on selected transitions detected in the incoming data signal. Theclock recovery circuit 650 includes a transition detect circuit 651,early/late detector 661, early/late counter 663, majority detector 665,interpolator 667 and reference loop 669. In the embodiment of FIG. 26,the transition detect circuit 651 asserts a transition detect signal 652(TDET) upon detecting a 01-to-10 or a 10-to-01 transition in theincoming data signal. More specifically, incoming data samples D_(N−1)and D_(N−2) are compared in exclusive-OR gate 653 to determine whether atransition has occurred, and signals D_(N) and D_(N−2) are compared inexclusive-NOR gate 657 to determine whether the D_(N) state matches theD_(N−2) state. The outputs of the exclusive OR and exclusive-NOR gates653, 657 are supplied to AND gate 659 which generates the transitiondetect signal 652. By this arrangement, the transition detect signal 652goes high if D[N:N−2]=010 or 101 (i.e., in response to a 01-to-10transition or a 10-to-01 transition). The transition detect signal 652is applied to a count enable input (CEN) of the early/late counter 663to enable an early/late count value to be incremented or decrementedaccording to the output of the early/late detector 661. In oneembodiment, the early late detector 661 is implemented by an exclusiveOR gate, and therefore asserts an early signal 654 (e.g., a logic highsignal) if the transition sample, T_(N), does not match data sampleD_(N), and a late signal (e.g., logic low signal 654) if the T_(N)matches D_(N). That is, if the transition sample is captured after thetransition from D_(N−1) to D_(N), the transition sample will match theD_(N) sample and thereby indicate that the edge clock signal transitionis late relative to the data signal transition. Conversely, if thetransition sample is captured before the transition from D_(N−1) toD_(N), the transition sample will not match the D_(N) sample and therebyindicate that the edge clock signal transition is early relative to thedata signal transition.

In one embodiment, the early/late counter 663 is initialized to a zerolevel, and is then incremented in response to an early signal (i.e.,from the early/late detector 661) and decremented in response to a latesignal. By this arrangement, the sign bit (e.g., the MSB) of theearly/late count maintained within the early/late counter 663 indicateswhether more early signals than late signals, or more late than earlysignals have been received from the early/late detector (i.e., the countvalue will underflow to a negative value if more late signals than earlysignals are detected). Accordingly, after a predetermined number oftransition detect assertions (or after a predetermined time), themajority detector 665 evaluates the sign of the early/late count andasserts an up/down signal 668 to the interpolator accordingly. Theearly/late count value may then be reset to zero in order to count asubsequent set of early/late signal assertions.

In one embodiment, the interpolator 667 maintains a digital control wordthat is incremented in response to a logic high up/down signal 668 anddecremented in response to a logic low up/down signal 668. The mostsignificant bits of the digital control word are used to select a pairof phase vectors from the set of N phase vectors 672 generated by thereference loop 669, and the least significant bits of the digitalcontrol word are used to interpolate between the selected pair of phasevectors. As the control word is incremented, the interpolation isincrementally shifted from a leading one of the phase vectors to alagging one of the phase vectors, thereby incrementally retarding thephase of the edge and sampling clock signals 610, 210. Conversely, asthe control word is decremented, the interpolation is incrementallyshifted toward the leading one of the selected phase vectors, therebyincrementally advancing the phase of the edge and sampling clock signals610, 210.

In one embodiment, the reference loop 669 is formed by a delay lockedloop (DLL) that receives a reference clock signal 670 and, in response,generates a plurality of phase vectors 672 that are phase distributedwithin a cycle time of the reference clock signal 670. Alternatively,the reference loop 669 may be a phase locked loop (PLL) that multipliesthe reference clock frequency to generate a plurality of phase vectors672 having a higher frequency than the reference clock frequency. Inanother alternative embodiment, the reference loop 669 may include aninternal timing reference generator (e.g., a ring oscillator or otherclock generating circuit) so that no reference clock signal 670 isrequired. Also, as discussed above, the interpolator 667 may generateany number of sampling clock and edge clock signals. For example, in adouble data rate system, the interpolator generates an edge clock signaland complement edge clock signal, and a sampling clock signal andcomplement sampling clock signal, the sampling clock signal being offsetfrom the edge clock signal by a quarter cycle (90 degrees) of the edgeclock signal. The quarter cycle offset may be achieved, for example, bya second interpolator that maintains a control word having a 90 degreedigital offset from the control word used to generate the edge clocksignal. Other techniques may be used to generate the edgeclock-to-sample clock offset in alternative embodiments. In a quad datarate system, the interpolator generates four edge clock signals and foursampling clock signals, the combined set of eight clock signals beingdistributed in phase through a cycle time of the edge clock signal(i.e., 45 degree increments between successive clock edges). Thisapproach may be extended to support virtually any data rate.

It should be noted that numerous changes may be made to the clockrecovery circuit of FIG. 26 without departing from the scope of thepresent invention. For example, in one alternative embodiment, theup/down signal is a two-bit signal in which the 00 state indicates ahold condition in which control word maintained within the interpolatoris not adjusted. In such an embodiment, the majority detector mayreceive the entire early/late count from the early/late counter, andoutput the up/down signal in the 00 state if the count value indicates abalanced reception of early and late detections (e.g., the early/latecount is zero). Alternatively, the majority detector may be omittedaltogether and the sign of the early/late count value output directly tothe interpolator to control the phase adjustment of the edge andsampling clock signals.

Returning to FIG. 25, it can be seen that the 11-to-01 transition occursbetween the 1+α and −1+α signal levels, and therefore crosses themidpoint between the centers of data eyes 628 and 630 (i.e., time T1) atthe +α threshold level. That is, the 11-to-01 transition crosses the +αthreshold coincidentally with the desired transition time of the edgeclock signal 610. Similarly, the 00-to-10 transition crosses the −αthreshold coincidentally with the desired transition time of the edgeclock signal 610. Moreover, the 11-to-01 and 00-to-10 transitions have afaster slew rate than the 10-to-01 and 01-to-10 transitions andtherefore tend to yield more accurate timing edges. That is, the signalpasses through the metastable region more quickly, thereby generatingless timing jitter. Accordingly, in one embodiment, additional edgecomparators are provided to generate transition samples at the +α and/or−α thresholds. Additional circuitry is also provided within the clockrecovery circuit 650 of FIG. 26 to detect the 11-to-01 and/or 00-to-10transitions and, in response, to update the early/late counter accordingto the corresponding transition samples. By this arrangement, theoverall number of data signal transitions that are used for clockrecovery is increased, thereby relaxing the transition density requiredin the incoming signal for clock recovery purposes.

In partial response receiver embodiments in which the ±α levels areadapted (i.e., as opposed to being initially calibrated or determinedempirically and one-time programmed), the ±α levels may initially beincorrect (e.g., started at 0) so that edge comparison based on the ±αlevels may be undesirable. In such a system, a zero-threshold edgecomparator (e.g., edge-sampling comparator 607 of FIG. 24) may be usedto recover the edge and sampling clock signals initially, withswitchover to ±α edge comparators occurring after the ±α levels aredetermined. Transition samples generated by zero-threshold edgecomparator may then be ignored, or the samples may continue to be used.

In a system that recovers edge and sampling clock signals based on ±αthreshold crossings, and that adapts ±α levels based on a differencebetween the 1+α and 1−α signal levels, a non-convergence of the ±αthreshold levels may occur in certain situations. Referring to bimodaldistribution shown in FIG. 27, it can be seen that as the sampling clockphase moves left (i.e., in the increasingly lagging direction), thedifference between signal levels 682 and 684 increases. Unfortunately,the increased difference between signal levels 682 and 684 yields largermagnitudes for the ±α threshold levels which, as can be seen in thediagram of FIG. 25, shifts the phase of the edge and sampling clocksignals further in the lagging direction (i.e., further to the left),thereby further increasing the clock phase error. If the clock phaseerror becomes too large, accurate data and edge samples are no longerreceived by the partial response receiver so that the ±α thresholdlevels will not converge to the desired setpoints.

Still referring to FIG. 27, one solution to the non-convergence problemis to limit the data samples used to adapt the ±α threshold levels tothose indicated by bolded lines 681 and 683. That is, in the case of asample for which D[N:N−1]=11, the +α level is updated only if thepreceding state was 10 (i.e., D[N−1:N−2]=10). Similarly, in the case ofa sample for which D[N:N−1]=10 the −α threshold level is updated only ifthe preceding sate was 01 (i.e., D[N−1:N−2]=01). By this arrangement, asthe edge and sampling clock signals begin to lag the desired phaseoffsets (i.e., T_(SAMPLE) moving to the left in the diagram of FIG. 27),the difference between the +α and −α threshold levels will decrease,thereby producing a counter-effect to shift the edge and sampling clocksignals in the leading direction. The level-adapting procedure describedin reference to FIG. 21 may be modified to accommodate the above changeby changing the evaluation in decision block 525 to D[N−1:N−3]=110 andchanging the evaluation in decision block 537 to D[N−1:N−3]=101. Notethat these sample patterns are illustrated in FIG. 27 as being patternsfor bits D[N:N−2]. In general, the data samples relied upon for leveladaptation may have any latency.

FIG. 28 illustrates an embodiment of a dual-mode, partial responsereceiver 700 with clock data recovery. The partial response receiver 700includes a dual-mode data receive/level sampling circuit 701, adaptivemodule 703, edge comparators 707, 708, 709, and clock recovery circuit705. The dual-mode data receive/level sampling circuit 701 and theadaptive module 703 operate in generally the same manner as thecircuitry described in reference to FIG. 22. That is, when the modeselect signal selects a 4-PAM mode of operation, the data receive andlevel sampling circuit samples incoming 4-PAM signals to generate an MSBand LSB per captured sample, and generates a data level error sample(C_(DLEV)) that allows the adaptive module to determine the full scaledifference between steady-state high and low signal levels. The +α, −αand zero threshold levels are then established by the adaptive module703 at the C_(α)=⅔ full-scale, C_(−α)=⅓ full-scale and C₀=½ full-scalelevels, respectively (other threshold levels may be used in alternativeembodiments). When the mode select signal selects a 2-PAM mode ofoperation, the data receive and level sampling circuit 701 generatesdata samples by selecting between partial-response comparators (i.e.,the comparators receiving the ±α threshold levels) and supplies theselected samples to the adaptive module 703 in pairs to enabledetermination of which of partial response state is reflected by theerror sample, E_(DLEV), generated by data level comparator (or datalevel comparators). The adaptive module 703 then adjusts the ±αthreshold levels and the zero threshold level according to the incomingdata signal levels determined by the data level comparator.

The edge comparators 707, 708 and 709 capture transition samplesT_(N−2)(+α), T_(N−2)(−α) and T_(N−2)(0), respectively, of the incomingdata signal in response to transitions of an edge clock signal, ECLK,and provide the transition samples to the clock data recovery circuit705. The 4-PAM/partial response data samples captured by the datareceive and level sampling circuit 701 (i.e., MSB/LSB in 4-PAM mode,D[N−2:N−3] in partial response mode) are also provided to the clock datarecovery circuit 705. The clock data recovery circuit selectivelyadjusts the phases of the edge clock signal 610 and sampling clocksignal 210 based on the transition samples and data samples.

FIG. 29 illustrates the possible signal transitions between successive4-PAM symbols. As shown, from each of four possible signal levels, theincoming data signal may transition to any of three other signal levels.For example, a signal level above +α (corresponding to data state 10)may transition to a signal level between +α and 0 (data state 11); asignal level between 0 and −α (data state 01); and a signal level below−α (data state 00). Examining the different possible transitions, it canbe seen that any transitions that cross all three threshold levels willcross the zero threshold level at the timing center, T1, between thedesired data sampling instants. Similarly, transitions that cross asingle threshold level will cross either the zero threshold level, the+α threshold level or the −α threshold level at T1. By contrast, anytransitions that cross two threshold levels, but not three, do not crossthe zero, +α or −α threshold levels at T1. Enumerating the differenttransitions that cross the zero, +α and −α threshold levels at T1 astransition types (1), (2) and (3), respectively, it can be seen thattype (1) transitions are those in which the LSB remains unchanged ateither ‘1’ or ‘0’, while the MSB changes state (i.e., (MSB_(N) xorMSB_(N−1)) & (LSB_(N) xnor LSB_(N−1))); type (2) transitions are thosein which the MSB remains high while the LSB changes state (i.e., MSB_(N)& MSB_(N−1) & (LSB_(N) xor LSB_(N−1))); and type (3) transitions arethose in which the MSB remains low, while the LSB changes state (i.e.,/MSB_(N) & /MSB_(N−1) & (LSB_(N) xor LSB_(N−1))). Thus, in oneembodiment, when the mode select signal 712 selects a 4-PAM mode ofoperation within the partial response receiver 700 of FIG. 28, the clockrecovery circuit adjusts the phase of the edge clock signal and samplingclock signal in response to the data samples generated by the datareceive and level sampling circuit 701 and the transition samplesgenerated by comparators 707, 708, 709 in accordance with the followingtable:

TABLE 1 Center Time Crossing Early/Late D_(N−1)(4P) D_(N)(4P) At:T_(N)(+α) T_(N)(−α) T_(N)(0) Count Adj. 00 01 −α X  0/1* X +1/−1 00 11 —X X X — 00 10 0 X X 0/1 +1/−1 01 00 −α X 0/1 X −1/+1 01 11 0 X X 0/1+1/−1 01 10 — X X X — 11 00 — X X X — 11 01 0 X X 0/1 −1/+1 11 10 +α 0/1X X +1/−1 10 00 0 X X 0/1 −1/+1 10 01 — X X X — 10 11 +α 0/1 X X −1/+1(*‘0/1’ means ‘0’ or ‘1’ and corresponds to the +1 or −1 adjustment tothe early/late count)

Still referring to FIG. 28, when the partial response receiver 700 isoperated in the partial response mode, the transitions of interest areas shown in FIG. 30. That is, a partial-response state transition from10-to-01 or 10-to-01 crosses the zero threshold level at the desirededge clock transition time, T1; a state transition 11-to-01 crosses the+α threshold level at T1; and a state transition from 00-to-10 crossesthe −α threshold level at T1. Enumerating the partial response modetransitions that cross the zero, +α and −α threshold levels at T1 astransition types (1), (2) and (3), respectively, it can be seen thattype (1) transitions are those in which the current data sample, D_(N),does not match the immediately preceding data sample, D_(N−1), which, inturn, does not match the twice-removed data sample, D_(N−2) (i.e.,(D_(N) xor D_(N−1)) & (D_(N−1) xor D_(N−2))); type (2) transitions arethose in which the current data sample, D_(N), is low, and the twoimmediately preceding data samples, D_(N−1) and D_(N−2), are high (i.e.,/D_(N) & D_(N−1) & D_(N−2)); and type three (3) transitions are those inwhich the current data sample, D_(N), is high, and the two immediatelypreceding data samples, D_(N−1) and D_(N−2), are low (i.e., D_(N) &/D_(N−1) & /D_(N−2)). Thus, in one embodiment, when the mode selectsignal 712 selects a partial response mode of operation within thepartial response receiver 700, the clock recovery circuit 705 adjuststhe phase of the edge clock and sampling clock signals 610, 210 inresponse to the data and transition samples generated by circuit 701 andcomparators 707, 708, 709 in accordance with the following table:

TABLE 2 Center Time Early/Late D[N − 1:N − 2] D[N:N − 1] Crossing At:T_(N)(+α) T_(N)(−α) T_(N)(0) Count Adj. 00 10 −α X 0/1 X +1/−1 01 00 — XX X — 01 10 0 X X 0/1 +1/−1 10 01 0 X X 0/1 −1/+1 10 11 — X X X — 11 01+α 0/1 X X −1/+1

Still referring to FIG. 30, if the +α and −α threshold levels areinitially set to zero ted by reference numeral 741, it can be seen thatthe edge clock alignment will initially to a point that is phase delayedrelative to the desired edge sampling point, T1. As the ±α progresstoward their ultimate setpoints at 742 and 743, however, the edge clockphase alignment will converge to the desired sampling point, T1.

FIG. 31 illustrates a transition detect circuit 725 and sample selectcircuit 740 that may be used within the clock recovery circuit 705 ofFIG. 28. The transition detect circuit receives the data sample pairgenerated by the data receive and level sampling circuit 701 andgenerates a transition detect signal 732 (TDET)in accordance with thestates of successive data sample pairs, the mode select signal 712(MSEL) and a transition select signal 728 (TS[3:1]). The transitiondetect circuit additionally generates component transition detectsignals 733 ₁, 733 ₂ and 733 ₃(i.e., TDET(0), TDET(+α) and TDET(−α),respectively) which are output to the transition select circuit 740. Thetransition select circuit includes AND gates 741 ₁, 741 ₂, and 741 ₃ togate the transition samples T_(N)(0), T_(N)(+α) and T_(N)(−α), accordingto the state of the corresponding transition select signals 733 ₁-733 ₃.The outputs of the AND gates 741 are input to OR gate 743 to generate anearly signal 734. The transition detect signal 732 is itself generatedby a logic OR combination of the component transition detect signals 733in OR gate 733. By this arrangement, if any of the component transitiondetect signals 733 is asserted (e.g., to a logic high state), thetransition detect signal 732 is asserted, and the state of thecorresponding transition sample T_(N) is output as the early signal 734.

The transition detect circuit 725 includes a set of combinatorial logiccircuits 727 ₁, 727 ₂ and 727 ₃ that generate type (1), type (2) andtype (3) transition detect signals 730 for both 4-PAM and partialresponse data states in accordance with the type (1), type (2) and type(3) transition types described in reference to FIGS. 29 and 30. In theembodiment of FIG. 31, combinatorial logic circuit 727 ₁, generates a4-PAM type (1) transition detect signal 730 _(1A) and a 2-PAM, partialresponse type (1) transition detect signal 730 _(1B) as follows:

Signal 730 _(1A)=(MSB_(N) xor MSB_(N−1)) & (LSB_(N) xnor LSB_(N−1)); and

Signal 730 _(1B)=(D_(N) xor D_(N−1)) & (D_(N−1) xor D_(N−2)).

Similarly, combinatorial logic circuits 727 ₂ and 727 ₃ generate 4-PAMtype (2) and type (3) transition detect signals 730 _(2A) and 730 _(3A),and 2-PAM, partial response type (2) and type (3) transition detectsignals, 730 _(2B) and 730 _(3B), as follows:

Signal 730 _(2A)=(MSB_(N) & MSB_(N−1)) & (LSB_(N) xor LSB_(N−1));

Signal 730 _(3A)=(/MSB_(N) & /MSB_(N−1)) & (LSB_(N) xor LSB_(N−1));

Signal 730 _(2B)=/D_(N) & D_(N−1) & D_(N−2); and

Signal 730 _(3B)=D_(N) & /D_(N−1) & /D_(N−2).

It should be noted that, in both partial-response mode and 4-PAM mode,two successive pairs of data samples are used within the transitiondetect circuit 725 (e.g., MSB/LSB_(N) and MSB/LSB_(N−1) in 4-PAM mode;D_(N)/D_(N−1) and D_(N−1)/D_(N−2) in partial response mode) to generatethe transition detect signals 730. One or more of the data sample pairsmay be buffered within a storage circuit within transition detectcircuit 725 or elsewhere within the dual mode receiver and madeavailable to the various combinatorial logic circuits 727. Also, thelatency of the data samples referred to in FIG. 31, though depicted asN, N−1 and N−2, may be any latency necessary to match the latency of thedata samples output from the data receive and level sampling circuit 701of FIG. 28.

Select circuits 729 ₁, 729 ₂ and 729 ₃ each have a first input portcoupled to receive a respective one of the 4-PAM type (1), type (2) andtype (3) transitions detect signals 730 _(A), 730 _(2A) and 730 _(3A),respectively, and a second input port coupled to receive a respectiveone of the 2-PAM, partial response type (1), type (2) and type (3)transition detect signals 730 _(1B), 730 _(2B) and 730 _(3B),respectively. The mode select signal 712 is coupled to a select input ofeach of the select circuits 729 so that, when the mode select signal 712is low to select a 4-PAM mode of operation, the 4-PAM transition detectsignals 730 _(1A), 730 _(2A) and 730 _(3A) are supplied to inputs ofrespective AND gates 731 ₁, 731 ₂ and 731 ₃, respectively. By contrast,when the mode select signal 712 is high, the 2-PAM, partial responsetransition detect signals 730 _(1B), 730 _(2B) and 730 _(3B) aresupplied to respective inputs of the AND gates 731. In the embodiment ofFIG. 31, the transition select signal 728 includes three componentsignals, TS[1], TS[2] and TS[3], that are input to AND gates 731 ₁, 731₂ and 731 ₃, respectively, to gate the generation of the componenttransition detect signals 733 ₁, 733 ₂, and 733 ₃. Thus, the transitionselect signal 728 may be used selectively enable the different types ofdata signal transitions to be used for clock recovery purposes. Forexample, if TS[3:1]=001, then detection of type (1) transitions (i.e.,crossing the zero-threshold at the desired time) is enabled, butdetection of type (2) and type (3) transitions (i.e., crossing the±αthresholds at the desired time) is disabled. When TS[3:1]=111, thendetection of all three types of transitions depicted in FIGS. 29 and 30is enabled. Other settings of the transition select signal may be usedto enable detection of the different transition types in othercombinations. In one embodiment, the transition select signal isgenerated according to a transition select value stored in aconfiguration circuit that may be run-time programmed (e.g., by a hostprocessor or other system control component). The transition selectvalue may alternatively be programmed into a non-volatile storagecircuit (e.g., at production time) or similar hardware configurablecircuit (e.g., fuse-blowing to establish desired configuration).

The transition detect signal 732 and early signal 734 generated by thetransition detect circuit 725 and transition select circuit 740 may beused to control the phase of one or more sampling clock signals and oneor more edge clock signals in the manner above in reference to FIG. 26.For example, in one embodiment the transition detect signal 732 isapplied to the count enable input of an early/late counter, and theearly signal 734 is applied to the up/down input. The transition detectsignal 732 and early signal 734 may alternatively be supplied to otherlogic circuitry that determines whether detected transitions occur earlyor late relative to an edge clock signal.

Numerous changes may be made to the transition detect circuit 725 andtransition select circuit 740 without departing from the spirit andscope of the present invention. For example, if the data signaltransitions to be used for clock recovery is a subset of the threetransition types shown in FIGS. 29 and 30, one or more component signalsof the transition select signal (and corresponding components of thecombinatorial logic 727, select circuitry 729 and gating circuitry 731,741) may be omitted. Moreover, if the transition types are fixed for agiven application (e.g., all three types of the transitions depicted inFIGS. 29 and 30, or any subset or single one thereof), the transitionselect signal may be omitted altogether along with the combinatoriallogic and gating circuitry for any unused types of transitions.

FIG. 32 illustrates the full set of transitions that may be detectedwhen the partial response receiver 700 of FIG. 28 is operated in thepartial response mode. In particular, transitions 750 and 751 (i.e.,transitions from 10-to 01 and from 01-to-00), designated type (4) andtype (5) transitions, respectively, can be seen to cross the nominal 1and −1 threshold levels (referred to herein as unity threshold levels),respectively, at the desired edge sampling point, T1. Note that, inthese transitions, the input signal slews substantially faster than inthe 01-to 10 and 10-to-01 transitions and therefore is a potentiallymore accurate (and/or more jitter-free) source of clock phaseinformation. Accordingly, in one embodiment, additional edge comparatorsare provided in the circuit of FIG. 28 (not shown), and correspondingadditional logic is added to the transition detect and select circuits(725, 740) of FIG. 31 to enable clock recovery based on type (4) and/ortype (5) transitions. Alternatively, in one embodiment, thezero-threshold comparator 709 of FIG. 28 is re-allocated to a unitythreshold comparator function after the ±α threshold levels areestablished (e.g., converge to the points 742 and 743 depicted in FIG.30). FIG. 33 illustrates an embodiment of a threshold select circuit 760that is used to select between control values C₀ and C₁ for the zero andunity threshold levels, respectively. Initially a threshold selectsignal 762 (LSEL) is supplied in a first state to multiplexer 761 (orother select circuit) to select the C₀ control value to be provided toedge comparator 709, thereby enabling the 10-to-01 and 01-to-10transitions to be used for clock recovery purposes as the ±α levels areadapted. After the ±α levels have reached convergence points (e.g.,begin to dither), the threshold select signal 762 is switched to analternate state to select the C control value to be used for generationof edge samples. The C₁ control value may be generated by the adaptivemodule 703 of FIG. 28 using the techniques discussed above in referenceto FIG. 22. The C₀ and/or C₁ control values may be buffered in registers763 and 765 as shown in FIG. 33 (or other storage circuits), or provideddirectly to the multiplexer 761 from the adaptive module.

Referring again to the partial response receiver 700 of FIG. 28, itshould be noted that the receiver may alternatively be a single-mode4-PAM circuit having the clock recovery function described above inreference to FIGS. 28, 29 and 31 (i.e., omitting the combinatorialcircuitry and mode select circuitry used to enable the partial responsemode). That is, the 4-PAM circuit may include data receive circuitry forcapturing a sequence of 2-bit data samples (or more bits per data samplein the case of M-PAM, M>4), and clock recovery circuitry that capturestransition samples at one or more of the 0, +α and −α threshold levels,and that selectively uses the transition samples to recover a samplingclock signal and edge clock signal according to the state of successivedata samples. Conversely, the circuitry necessary to enable the 4-PAMmode of operation may be omitted to provide a partial-response receiverwith clock recovery circuitry that captures transition samples at one ormore of the 0, +α and −α threshold levels, and that selectively uses thetransition samples to recover a sampling clock signal and edge clocksignal according to the state of successive data samples.

In another alternative embodiment, the partial response receiver 700 ofFIG. 28 may include a third, non-partial-response operating mode inwhich binary signal reception and clock recovery are performed withoutregard to partial response. That is, the +α and −α comparators withinthe data sampling circuit and edge sampling circuit may be disabled, andthe zero-threshold comparators within the data and edge samplingcircuits being used to sample data and edges in the incoming signal.Alternatively, in the non-partial-response mode, the ±α threshold levelsmay be set to zero such that all the comparators within the data andedge sampling circuits perform a zero-threshold comparison. In yet otherembodiments, the partial response receiver 700 of FIG. 28 may beoperable in either a 4-PAM or binary mode, with partial response signaldetection being separately enabled in either mode. That is, the partialresponse receiver 700 may be operated in either a binary non-partialresponse mode; a binary partial response mode; a 4-PAM non-partialresponse mode; or a 4-PAM partial response mode. Moreover, the number ofleast-latent data samples included in the partial response may beextended to any number. More generally, the receive circuit 700 may beoperated in PAM modes ranging from M-PAM to (M−X)-PAM (where M is anyinteger and X is any integer less than M−1), and may be operated with orwithout regard to partial response in each PAM mode. Circuitry tosupport extended-bit partial response operation, and 4-PAM partialresponse operation is discussed below in greater detail.

Extending the Partial Response Compensation to Include Additional DataHistory

Thus far, partial response receivers have been described in terms of abimodal distribution based on the current symbol, D_(N), and thepreceding symbol, D_(N−1). In alternative embodiments, partial responsesto any number of preceding symbols may be accounted for in thegeneration of data samples. FIG. 34, for example, illustrates adistribution 770 of data-plus-partial-response signal levels in a systemin which the two most recently received symbols are the primary sourceof residual channel response (i.e., the channel memory includes apartial response, α, to the immediately preceding symbol, D_(N−1), and apartial response, β, to the twice-removed preceding symbol, D_(N−2)).For purposes of simplicity, the signal level constellation shown in FIG.34 assumes a substantially equal distribution of the eight partialresponse signal levels: 1+α+β, 1+α−β, 1−α+β, 1−α−β, −1+α+β, −1+α−β,−1−α+β, and −1−α−β. Other signal distributions may result depending uponthe channel characteristics. As shown, the constellation may be viewedas defining four signal ranges 771 ₁-771 ₄, each range being centeredabout one of the four possible partial response levels, α+β, α−β, −α+β,and −α−β.

FIG. 35 illustrates a partial response receiver 800 that operates inaccordance with the partial response states shown in FIG. 34. Fourcomparators 801 ₁, 801 ₂, 801 ₃ and 801 ₄ are provided to compare thesignal level of an incoming symbol, D_(N), against the four partialresponse threshold levels α+β, α−β, −α+β and −α−β, respectively. Theoutputs of each of the comparators 801 are stored in a first stage ofstorage circuits 802 ₁-802 ₄ (e.g., in response to a sampling clocksignal, not shown), with the sample values that correspond to the +αthreshold levels (i.e., D_(N−1)(α+β) and D_(N−1)(α−β)) being supplied toa first select circuit 810 and the sample values that correspond to the−α threshold levels (i.e., D_(N−1)(−α+β) and D_(N−1)(−α−β)) beingsupplied to a second select circuit 812. Each of the first and secondselect circuits 810 and 812 selects between its respective pair of inputsamples according to the state of a previously resolved sample value,D_(N−3), stored in storage circuit 820. More specifically, if theresolved sample value, D_(N−3), is a ‘1’, then β, the partial responseto D_(N−3), is a positive value and the select circuits 810 and 812select the sample values D_(N−1)(α+β) and D_(N−1)(−α+β), respectively,that correspond to the positive β state. If the D_(N−3) sample is alogic ‘0’ value, then β is a negative value and the select circuits 810and 812 select the sample values D_(N−1)(α−β) and D_(N−1)(−α−β) thatcorrespond to the negative β state. The D_(N−1) sample values selectedby the select circuits 810 and 812 correspond to the +α and −α partialresponse states, respectively, and are stored in storage circuits 814and 816 to provide D_(N−2) sample values D_(N−2)(α) and D_(N−2)(−α). TheD_(N−2) (α) and D_(N−2)(−α) samples are output from the storage circuits814 and 816 to respective inputs of select circuit 818. The state of theD_(N−3) sample value indicates the sign of the partial responsecontribution in the D_(N−2) signal. That is, if D_(N−3)=1, a ispositive, and if D_(N−3)=0, a is negative. Accordingly, the D_(N−3)sample value is supplied to a select input of select circuit 818 toselect either D_(N−2)(+α) or D_(N−2)(−α). Thus, the partial responsereceiver 800 of FIG. 35 simultaneously compares the incoming data signalagainst four different partial response thresholds, then selects one ofthe four sample values to be the output sample value (D_(N−3)) based onthe previously resolved state of the a and β a partial responsecomponents. Partial response components from more than two previouslyreceived symbols may be accommodated in a similar manner by increasingthe number of comparators to match the number of partial response levelsto be resolved, and then selecting the desired partial response sampleon the basis of the partial response components indicated by apreviously resolved sample value.

FIG. 36 illustrates a partial response receiver 840 according to anotherembodiment of the invention. The partial response receiver 840 includesa partial response receive circuit 800, level sampler 841 and adaptivemodule 850. The partial response receive circuit 800 operates generallyas described in reference to FIG. 35 to generate sample value D_(N−3),which is supplied to the adaptive module 850. The level sampler 841operates in generally the same manner as the level sampling circuit ofFIG. 14, except that eight comparators 842 ₁-842 ₈ are provided to errorsamples for each of the eight possible partial response levels depictedin FIG. 34 (i.e., E₀₀₀, E₀₀₁, . . . , E₁₁₀, E₁₁₁). The level sampleradditionally includes storage circuits 844 to latency-align the errorsamples generated by comparators 842 with the data sample D[N−3]generated by the partial response receive circuit 800. The adaptivemodule 850 operates similarly to the adaptive module described inreference to FIGS. 14-16, except that control values for each of theeight partial response threshold levels applied within comparators 842are updated in response to reception of a corresponding one of the eightpossible data patterns for sample values D[N−3:N−5] (note that datasamples D_(N−4) and D_(N−5) may be generated by buffering data samplesreceived from the partial response receive circuit 800 within theadaptive module 850). That is, when D[N−3:N−5]=111, control valueC_(1+α+β) is incremented or decremented according to the state of theerror sample, E₁₁₁, generated by comparator 842 ₈; when D[N−3:N−5]=110,C_(1+α+β) is incremented or decremented according to the state of errorsample E₁₁₀, and so forth to error sample E₀₀₀ which is used to updateC_(−1−α−β) when D[N−3:N−5]=000.

In one embodiment, each of the four threshold control values supplied tothe partial response receive circuit 800 (i.e., C_(α+β), C_(α−β),C_(−α+β) and C_(−α−β)) are generated by averaging respective pairs ofthe control values output to the level sampler 841. That is:C _(α+β)=(C _(1+α+β) +C _(−1+α+β)/)2;C _(α−β)=(C _(1+α−β) +C _(−1+α−β))/2;C _(−α+β)=(C _(1−α+β) +C _(−1−α+β))/2; andC _(−α−β)=(C _(1−α−β) +C _(−1−α−β))/2

Alternatively, the threshold control values supplied to the partialresponse receive circuit 800 may be generated by halving the differencebetween respective pairs of the control values output to the levelsampler 841 (e.g., C_(α+β)=(C_(1+α+β)−C_(1−α−β))/2).

Partial Response, Multi-PAM Receiver

The partial response operation described above in the context of binarysignaling may be extended to multi-PAM receivers. FIG. 37, for example,illustrates a constellation of possible signal levels in a 4-PAMsignaling system. More specifically, assuming that a given data symbolhas one of the four levels +3, +1, −1 or −3 centered about a nominalzero value, and that the primary partial response is due to theimmediately preceding symbol, then sixteen partial response statesbecome possible. That is, a symbol nominally transmitted at level +3 mayhave any of the states 3+3α, 3+α, 3−α, or 3−3α when observed at thereceiver, depending on whether the signal level for the preceding symbolwas +3, +1, −1 or −3. Similarly, a symbol nominally transmitted atlevels +1, −1 or −3 may have any of the four states that correspond tothe additive or subtractive effect of the preceding symbol level. Asshown in FIG. 37, by grouping the possible partial response statesaccording to the signal level of the preceding symbol, it can be seenthat if the preceding symbol was a +3, the subsequently transmittedsymbol will have a signal level that is offset by 3α and therefore thatranges from −3+3α to +3+3α. Similarly, if the preceding symbol was a +1,the subsequently transmitted symbol will have a signal level that isoffset by 1α, and so forth for preceding symbols at −1 (offset =−1α) and−3 (offset =−3α). Accordingly, by providing four 4-PAM receive circuitseach having thresholds offset by respective one of the 3α, +α, −α and−3α partial response levels, a partial response mode of operation may beenabled within a 4-PAM receiver.

FIG. 38 illustrates a 4-PAM partial response receiver 875 according toan embodiment of the invention. The receiver 875 includes four 4-PAMreceive circuits 877 ₁-877 ₄ (i.e., each capable of resolving the signallevel of an incoming symbol into one of four possible two-bitcombinations) each having a threshold level that is offset according toone of the four partial response levels (3α, α, −α, or −3α). In the caseof receive circuit 877 ₄, for example, the 3α offset (corresponding to aprior symbol level of 10) is applied to each of the internal levelsgenerated to resolve the 4-PAM signal. That is, instead of comparing theincoming signal with levels of −2, 0, and 2, the incoming signal iscompared with −2+3α, 3α and 2+3α (i.e., threshold values between the3+3α, 1+3α, −1+3α, −3+3α constellation depicted in FIG Similarly, 4-PAMreceive circuit 877 ₃ compares the incoming signal with thresholdsoffset by α (i.e., −2+α, α and 2+α), comparator 877 ₂ compares theincoming signal with thresholds offset by −α(i.e., −2−α, −α, and 2−α)and comparator 877 ₁ compares the incoming signal with thresholds offsetby −3α (i.e., −2−3α, −3α and 2−3α). By this arrangement, the incomingsignal is resolved into four 2-bit sample values according to each ofthe four possible partial responses to the preceding symbol. Thepreceding symbol, MSB/LSB[N−1], having been selected according to thestate of MSB/LSB[N−2], is stored in storage element 881 and applied tothe select input of select circuit 879 to select the one of the foursample values generated in accordance with the incoming signal level.While the partial response operation has been described in terms of a4-PAM receiver that enables partial response selection based on a singlepreceding symbol, the circuits and operation described may be extendedto other PAM levels (e.g., 8-PAM, 10-PAM, etc.) and/or to include thepartial response to additional preceding bits.

FIG. 39 illustrates an input circuit 885 that includes multiple partialresponse receivers 887 ₁-887 _(M) according to an embodiment of theinvention. Each of the partial response receivers 887 ₁-887 _(M) iscoupled to a respective one of signal lines 883 ₁-883 _(M) to receive acorresponding one of data signals D₁-D_(M). In one embodiment, each ofthe data signals D₁-D_(M) is a serial stream of data values (e.g., dataeyes) that is sampled within the corresponding partial response receiver887 to generate a corresponding one of sample values, S₁-S_(M). Thesampled values S₁-S_(M) are delayed relative to the corresponding datavalues by one or more symbol times, i, according to the latencyintroduced by the partial response receiver 887. Thus, during the symboltime in which data value D_(M)[N] is valid at the input to partialresponse receiver 887 _(M), the partial response receiver outputs samplevalue S_(M)[N-i]. Partial response receivers 887 ₁-887 _(M−1) similarlyoutput respective sample values S₁[N−i]−S_(M−1)[N−i] during the symboltimes in which data values D₁[N]−D_(M−1)[N] are valid.

In one embodiment, each of the signal lines 893 ₁-893 _(M) forms aunidirectional or bidirectional point-to-point signaling link between afirst IC device that includes input circuit 885 and a second IC device.In the case of a bi-directional link, the first IC device mayadditionally include transmit circuitry coupled to the signal lines 893,and the second IC device may include a counterpart input/outputcircuitry having an input circuit similar to input circuit 885. Thefirst and second IC devices may be in separate IC packages coupled to acommon substrate (e.g., circuit board) or coupled to differentsubstrates (e.g., coupled to respective daughterboards with signal paths893 ₁-893 _(M) formed in part by signal traces on the daughterboards andon a backplane, or the first IC device coupled to a daughterboard andthe second IC device coupled to a motherboard). Alternatively, the firstand second IC devices may be included within the same IC package (e.g.,a multi-chip module, chip-stack package, paper-thin package orcombination of integrated circuit dice within a package). In anotherembodiment, each of the signal lines 893 ₁-893 _(M) forms aunidirectional or bidirectional multi-drop signaling link coupled tothree or more IC devices, with the IC devices or any subset thereofbeing disposed on the same or different substrates, and within the sameor different IC packages.

In either the point-to-point or multi-drop embodiments, the signal lines893 ₁-893 _(M) may constitute a parallel signaling path 894 fortransmission of multi-bit data values. For example, each of the samplevalues S₁-S_(M) for a given symbol time (i.e., corresponding to datavalues D₁-D_(M) recovered from path 894) may represent a unified digitalvalue (in which S_(M) is a most significant bit (MSB) and sample S₁ is aleast significant bit (LSB), M being any integer value greater than one.Also, subsets of bits within the sample values S₁-S_(M) may constituterespective unified digital values (e.g., S₁-S_(X) being a first valueordered from LSB to MSB, S_(X+1)-S_(Y) being a second data value orderedfrom LSB to MSB, and so forth to S_(Z+1) to S_(M) which constitute alast data value ordered from LSB to MSB). Further, any number ofsuccessively received samples (i.e., corresponding to a packetizedtransmission of values) may form a unified digital value. For example,S₁[N−i]−S_(M)[N−i], S₁[N−i−1]−S_(M)[N−i−1], . . . ,S₁[N−i−Q]−S_(M)[N−i−Q] may form a unified digital value having a totalof Q×M bits (Q being any integer value greater than one) in which onebit is an LSB and one bit is an MSB. Alternatively, a set ofsuccessively received samples may form multiple unified digitizedvalues, as when bits of a first digitized value are generated insuccessive symbol times by partial response receivers 887 ₁-887 _(x),bits of a second digitized value are generated in successive symboltimes by partial response receivers 887 _(X+1)-887 _(Y), and so forth.

In the embodiment of FIG. 39, each partial response receiver 887includes a partial response receive circuit 888, level sampler 889,adaptive module 890, edge sampler 891 and clock data recovery circuit892. The partial response receive circuit 888 may be a multi-modereceive circuit as described above (i.e., capable of being switchedbetween a multi-level signaling mode and a partial-response mode), ormay be dedicated to partial-response operation. Also, the partialresponse receive circuit may operate on incoming binary or multi-levelsignals (e.g., 2-PAM, 4-PAM, 8-PAM, 10-PAM, etc.) having any data rate(e.g., single data rate, double data rate, quad data rate, octal datarate, decade data rate, etc.). Thus, the partial response receivecircuit 888 may be implemented by any of the partial response receivecircuit embodiments described herein. Similarly, the level sampler 889and adaptive module 890 may be implemented by any of the level samplerand adaptive module embodiments described herein. For example, thoughdepicted as outputting a single error sample, C_(DLEV), the levelsampler 889 may output multiple error samples to the adaptive module890, and the adaptive module 890 may output any number of control valuesto the level sampler 889, partial response receive circuit 888 and edgesampler 891 (e.g., as shown in FIGS. 14, 28 and 36) in addition to orinstead of C_(DLEV), C_(α), /C_(α) and C₀. Also, though depicted in FIG.39 as recovering a sample clock signal (SCLK) and edge clock signal(ECLK), the edge sampler 891 and clock data recovery circuit 892 mayrecover any number of clock signals as described above in reference toFIGS. 24 and 26 (e.g., for use in multi-data rate embodiments). Further,the edge sampler 891 and clock data recovery circuit 892 may recoverphase information (i.e., for adjusting the phase of the recovered clocksignals) from any or all transitions of the incoming data signal 893 asdescribed above. The edge sampler 891 and clock data recovery circuit892 may be omitted from the partial response receiver 887 in anembodiment in which clock signals, strobe signals or other timingsignals are provided by another source, such as an external or internalclock generator, or separate clock recovery circuit.

FIG. 40 illustrates an input circuit 895 according to an alternativeembodiment of the invention. The input circuit 895 is similar to theinput circuit 885 of FIG. 39 (and may be implemented in all thealternative embodiments described in reference to FIG. 39), except thatthe circuitry used to generate partial response thresholds (orrepresentative control values) and sampling clock signals is sharedamong multiple partial response receive circuits 888 ₁-888 _(M). Thatis, a single level sampler 889 and corresponding adaptive module 890 areused to generate control values C_(α), /C_(α) and C₀ (and additional ordifferent threshold control values in multi-level signaling embodimentsand embodiments that do not include clock recovery circuitry) that aresupplied to each of the partial response receive circuits 888 ₁-888_(M), and a single edge sampler 891 and corresponding clock datarecovery circuit 892 are used to generate the sampling clock signal(SCLK) that is supplied to each of the partial response receive circuits888 ₁-888 _(M). Multiple sampling clock signals may be generated andshared among the partial response receive circuits 888 ₁-888 _(M) inmulti-data rate embodiments. By sharing the circuitry for generatingcontrol thresholds and/or the circuitry for generating sampling clocksignals, the amount of circuitry within the input circuit 895 issubstantially reduced relative to input circuit 885, reducing productionand operational cost of the host integrated circuit device (i.e., due toreduced die area consumption, layout complexity, test and verificationtime, power consumption, etc.). Note that, in an embodiment in which thesampling clock signal is provided by circuitry other than clock datarecovery circuitry (e.g., external clock source, internal clockgenerated), the edge sampler 891 and clock data recovery circuit 892 maybe omitted. Alternatively, programmable selection of the clock sourcemay be used to select either the recovered sampling clock (i.e.,recovered by operation of the edge sampler 891 and clock data recoverycircuit 892) or another clock source to provide a sampling clock signalto the partial response receive circuits 888 ₁-888 _(M). As in theembodiment of FIG. 39, the signal lines 893 ₁-893 _(M) may be used todeliver distinct serial transmissions, or transmission of related groupsof bits (e.g., the M bits received during a given symbol timerepresenting one or more multi-bit digital values) as, for example, inthe case of a parallel bus or any other signaling system in which thethreshold control values and/or timing control signals are expected tobe substantially similar (e.g., substantially same thresholds and/orclock phases) across multiple signaling paths. Also, while a singlelevel sampler 889 and corresponding adaptive module 890 are depicted inFIG. 40 as generating threshold control values C_(α), /C_(α) and C₀ foreach of the partial response receive circuits 888 ₁-888 _(M), in analternative embodiment, multiple level sampler/adaptive module pairs maybe used to generate threshold control values for respective subsets ofthe partial response receive circuits 888 ₁-888 _(M). Respective edgesampler/clock data recovery circuit pairs may similarly be used togenerate sampling clock signals for corresponding subsets of partialresponse receive circuits 888 ₁-888 _(M).

Still referring to FIG. 40, the partial response receive circuits 888₁-888 _(M) may be implemented by any of the embodiments described hereinincluding, without limitation, the embodiments described as described inreference to FIGS. 17-19. Also, in embodiments in which a preamplifier(e.g., preamplifier 401 of FIG. 17) or offset control circuit (e.g.,offset control circuit 440 of FIG. 19) is used to adjust the effectivethreshold of a differential comparator above or below a common mode,such preamplifier or offset control circuit may be shared among multiplepartial response receive circuits 888, thereby reducing the circuitrywithin the partial response receive circuits 888 ₁-888 _(M) to that of asampling circuit (e.g., sampler 425 of FIG. 17 or sampler 452 of FIG.19). Respective preamplifiers (or offset control circuits) may also beshared among subsets of the partial response receive circuits 888 ₁-888_(M).

Transmit-Side Equalization in System with Partial Response Receiver

FIG. 41 illustrates a signaling system 950 having a transmit device 951and receive device 953 coupled to one another via a signaling path 122.The transmit device 951 includes an equalizing transmit circuit 957 andtap control logic 955, and the receive device 953 includes a partialresponse receive circuit (which may, for example, be a multi-modecircuit or any other of the partial response receive circuits disclosedherein), level sampler 973 and adaptive module 975. Both the transmitdevice 951 and receive device 953 may include numerous other circuitsnot shown in FIG. 41 (e.g., application logic, additional transmitcircuits and/or receive circuits, etc.). The equalizing transmit circuit957 includes a number of output drivers 961 ₀-961 _(N−1) to drive asymbol onto signaling path 122 in response to a transmit timing signal(not shown) and in accordance with a set of weighting values W₀-W_(N−1)and the state of data values D₀-D_(N−1). Each of the data valuesD₀-D_(−(N−1)) is stored within a shift register 959 (or similar storagecircuit) and includes a number of constituent bits in accordance to thenumber of bits encoded into each symbol transmission on the signalingpath 102. For example, in a 4-PAM system, each of the data values,D₀-D_(−(N−1)), includes two constituent bits. More generally, in a M-PAMsystem, each of the data values includes log₂(M) constituent bits. Aftereach symbol transmission, the contents of the shift register 959 areshifted forward so that a new data value is shifted into position D₀,and the data value at position D_(−(N−1)) is overwritten by the datavalue previously at position D_(−(N−2)).

The weighting values W₀-W_(N−1) are generated by the tap control logic955 and are used to establish the signal drive strength of each of theoutput drivers 961. In one embodiment, the output driver 961 ₀ drivesthe symbol to be transmitted during a given symbol time in accordancewith weighting value W₀, and the output drivers 961 ₁-961 _(N−1)constitute a set of post-tap drivers for equalizing the output signalaccording to the post-tap data values stored within positionsD⁻¹-D_(−(N−1)) of the shift register 959 and the weighting valuesW₁-W_(N−1). Though not shown in FIG. 41, pre-tap output drivers may alsobe provided to equalize the output signal according to pre-tap datavalues (and pre-tap weighting values), and yet other output drivers maybe provided to cancel cross-talk from neighboring signal lines. Also,one or more of the output drivers 961 may be dynamically or staticallyallocated between pre-tap equalization, post-tap equalization andcross-talk cancellation functions, for example, through configurationregister settings or externally received configuration control signals.

The partial response receive circuit 971 includes R compare circuits togenerate respective partial response values according to the number ofexpected partial response signal levels. For example, in a system inwhich the least-latent symbol (i.e., the most recently received symbol,N−1−i, relative to incoming symbol N−i) is the only expected contributorto the partial response, two compare circuits are provided to generatepartial response values according to the two possible states ofleast-latent symbol (i.e., R=2). In a system in which the K mostrecently received symbols are expected to contribute to the partialresponse, 2^(K) compare circuits are provided to generate respectivedata samples based on comparisons with the K partial response thresholdlevels (C_(PR)) generated by the adaptive module 975.

In one embodiment, the tap control logic 955 of transmit device 951generates the weighting values W₀-W_(N−1) in accordance with the numberof symbols desired to contribute to the partial response observed at thereceive device 953. Referring to FIG. 42, for example, in a system inwhich the partial response is to be limited to the least latent symbol,D⁻¹ (i.e., K=1), the tap control logic 955 generates weighting valuesW₀-W_(N−1) to equalize the dispersion-type ISI produced by symboltransmissions after D⁻¹ and prior to D₀. That is, referring to the rawpulse response shown in FIG. 42, the partial response of the signalingpath 122 to symbols D₊₁, and D⁻² to D_(−(N−1)) is equalized byappropriate settings of the weighting values applied to a pre-tap outputdriver (not shown in FIG. 41) and post-tap output drivers 961 ₂-961_(N−1). By this arrangement, the equalized signal observed by thereceive device 953 (i.e., equalized for partial response as depicted inFIG. 42) includes a contribution from the symbol to be received, D₀, anda partial response to the immediately preceding symbol, D⁻¹, with thepartial response to all other transmissions being attenuated (e.g., to anegligible or substantially reduced level). In alternative embodiments,the tap control logic 955 may enable partial response to one or moreadditional symbols to remain in the signal observed by the receivedevice 953 depending on the desired number of partial responsecontributors. Also, in a multi-mode circuit in which a partial responsereceive circuit may optionally be operated in a non-partial responsemode, the tap control logic 955 may generate weighting values W₀-W_(N−1)as necessary to cancel (or reduce) the partial response to post-tap andpre-tap data values.

Still referring to FIG. 41, the level sampler 973 within the receivedevice 953 operates generally as described above (e.g., in reference tothe various embodiments described in reference to FIGS. 14-15 and 20-21)to compare the incoming signal with a data level threshold and generateerror samples (E_(DLEV)) that are used to adjust the data levelthreshold(s), C_(DLEV), and the partial response thresholds, C_(PR). Asdiscussed above, the data level threshold and/or the partial responsethresholds, C_(PR), may be digital values that are used to establish thedesired threshold levels within compare circuits of the partial responsereceive circuit 971 and the level sampler 973, or any other types ofcontrol values (e.g., analog voltage and/or current levels that arethemselves the threshold levels).

As discussed above in reference to FIG. 15, the adaptive module mayinclude filtering circuitry to filter the error samples generated by thelevel sampler 973 such that the threshold levels generated by theadaptive module are updated based on multiple error samples, rather thanon a sample-by-sample basis. For example, in one embodiment, theadaptive module includes a finite impulse response (FIR) filter toupdate the threshold levels based on a succession of error samplesapplied to the filter. Alternatively, the adaptive module may include aninfinite impulse response filter (IIR) or any other type of filter forfiltering the error samples.

In one embodiment, the data level error samples, E_(DLEV), generated bythe level sampler 973 are returned to the transmit device 951 via a backchannel 968 (e.g., out-of-band signaling, passage via an intermediarydevice such as the controller 141 of FIG. 3, or other communication paththat does not consume otherwise usable bandwidth on the signaling path122). The error samples are received within the tap control logic 955and used to adjust the tap weights, W₀-W_(N−1), as necessary to reduceISI (or other sources of distortion) resulting from transmission ofsymbols other than symbols desired to contribute to the partialresponse. Referring to FIG. 43, for example, after the data levelthreshold generated by the adaptive module 975 converges to a nominaldata level 985 (DLEV_(NOM)), further jitter detected in the signal datalevel 986 may be treated as residual ISI due resulting from transmissionof symbols other than symbols desired to contribute to the partialresponse. The tap control logic may increase and decrease the weights ofthe post-tap output drivers and/or pre-tap output drivers as necessaryto reduce the residual ISI. In alternative embodiments, updates to thetap weights themselves (e.g., values to be added and/or subtracted fromweights W₀-W_(N−1) or a subset thereof) may be determined within thereceive device (or another device such as controller 141 of FIG. 3) andprovided to the transmit device for application within the tap controllogic 955. In yet other embodiments, one or more replacement tap weightsmay be determined within the receive device (or other device such ascontroller 141 of FIG. 3) and provided to the transmit device forapplication in place of corresponding tap weights W₀-W_(N−1).

Section headings have been provided in this detailed description forconvenience of reference only, and in no way define, limit, construe ordescribe the scope or extent of such sections. Also, while the inventionhas been described with reference to specific embodiments thereof, itwill be evident that various modifications and changes may be madethereto without departing from the broader spirit and scope of theinvention. Accordingly, the specification and drawings are to beregarded in an illustrative rather than a restrictive sense.

1. A receiver circuit comprising: a plurality of sampling circuits, eachof the plurality of sampling circuits to sample a signal and generate asample value that indicates whether the signal exceeds a respectivethreshold level; a select circuit to receive the sample values from theplurality of sampling circuits and to select, based at least in partupon at least one previously selected sample value, one of the samplevalues as a currently selected sample value; and a threshold generationcircuit to receive the sample values from the plurality of samplingcircuits and to determine at least one data-conveying level of thesignal based on the sample values and adaptively generate the thresholdlevels based at least in part upon one or more previously selectedsample values and the at least one data-conveying level of the signal.2. The receiver circuit of claim 1, wherein the threshold generationcircuit generates the threshold levels during a scoping mode.
 3. Thereceiver circuit of claim 1, wherein the threshold generation circuitcomprises: a shift register having a plurality of storage elements toshift selected sample values through each of the plurality of storageelements; a second select circuit to select a subset of the plurality ofstorage elements; and an equalizer circuit to receive a subset of theselected sample values from the subset of the plurality of storageelements.
 4. The receiver circuit of claim 3, wherein the equalizercircuit generates an equalizing signal based on the subset of theselected sample values.
 5. The receiver circuit of claim 4, wherein eachof the plurality of sampling circuits comprises an input to receiveincoming symbols, and wherein the equalizer circuit outputs theequalizing signal to the input of the sampling circuitry to affectdeterminations of incoming symbol amplitudes.
 6. The receiver circuit ofclaim 3, further comprising a transmit circuit to generate an outputsymbol during a first time interval, the output symbol having anamplitude based, at least in part, on a transmit data value thatcorresponds to an output symbol generated during a time interval priorto the first time interval.
 7. The receiver circuit of claim 1 whereinthe threshold generation circuit comprises circuitry to determine the atleast one data-conveying level of the signal by adapting a firstthreshold level of the threshold levels to match an expecteddata-conveying level of the signal.
 8. The receiver circuit of claim 7wherein the circuitry to adapt a first threshold level to match theexpected data-conveying level of the signal comprises circuitry todetermine whether a level of the signal is greater than or less than thefirst threshold level and to increase or decrease the first thresholdlevel accordingly.
 9. The receiver circuit of claim 7 wherein thecircuitry to adapt a first threshold level to match an expecteddata-conveying level of the signal comprises circuitry to adapt a firstdata-level threshold to match a level of the signal that corresponds toa first pattern of bits conveyed sequentially in the signal.
 10. Thereceiver circuit of claim 9 wherein the circuitry to adapt a firstdata-level threshold to match a level of the signal that corresponds toa first pattern of bits comprises circuitry to adapt a second data-levelthreshold to match a level of the signal that corresponds to a secondpattern of bits conveyed sequentially in the signal.
 11. The receivercircuit of claim 10 wherein the circuitry to adapt a first data-levelthreshold and the second data-level threshold comprises circuitry toadapt the first and second data-level thresholds simultaneously,including first and second comparator circuits to compare the signal tothe first and second data-level thresholds, respectively.
 12. Thereceiver circuit of claim 10 wherein the circuitry to adapt the firstdata-level threshold and the second data-level threshold comprisescircuitry to update the first and second data-level thresholdssequentially, including a shared comparator circuit to compare thesignal to the first and second data-level thresholds at respectivetimes.
 13. The receiver circuit of claim 10 wherein the thresholdgeneration circuitry further comprises circuitry to generate, based onthe first and second data-level thresholds, a decision threshold fordistinguishing between data-conveying levels of the signal, the decisionthreshold being compared with the signal within at least one of theplurality of sampling circuits to generate the sample value.
 14. Areceive circuit comprising: a compare circuit to generate at least twosample values of an input symbol, each sample value having either afirst state or a second state according to whether the input symbolexceeds a respective threshold level; and a decision circuit to generatea received data value, the decision circuit being operable in apartial-response mode to select, based at least in part on a previouslyselected sample value, a sample value from the at least two samplevalues to be the received data value, the decision circuit further beingoperable in a non-partial-response mode to select a predetermined one ofthe at least two sample values to be the received data value.
 15. Thereceive circuit of claim 14, wherein the decision circuit further beingoperable in a third mode to generate the received data value having amost significant bit based at least in part on the state of a first ofthe at least two sample values and a least significant bit based atleast in part on the state of a second of the at least two samplevalues.
 16. The receive circuit of claim 14, wherein the compare circuitgenerates the at least two sample values based at least in part on atleast one previously received data value.
 17. The receive circuit ofclaim 14, wherein the decision circuit further being operable in thepartial-response mode to select, based at least in part on at least twopreviously selected sample values, a sample value from the at least twosample values to be the received data value.
 18. A receive circuitcomprising: a compare circuit to generate at least two sample values ofan input symbol, each sample value having either a first state or asecond state according to whether the input symbol exceeds a respectivethreshold level, the compare circuit to generate the at least two samplevalues based at least in part on at least one previously received datavalue; and a decision circuit to generate a received data value, thedecision circuit being operable in a first mode to select, based atleast in part on a previously selected sample value, a sample value fromthe at least two sample values to be the received data value, thedecision circuit further being operable in a second mode to generate thereceived data value having a most significant bit based at least in parton the state of a first of the at least two sample values and a leastsignificant bit based at least in part on the state of a second of theat least two sample values.
 19. The receive circuit of claim 18, whereinthe decision circuit further being operable in a third mode to selecteither a first of the at least two sample values or a second of the atleast two sample values to be the received data value.
 20. The receivecircuit of claim 18, wherein the decision circuit further being operablein the first mode to select, based at least in part on at least twopreviously selected sample values, a sample value from the at least twosample values to be the received data value.
 21. A receive circuitcomprising: a compare circuit to generate at least two sample values ofan input symbol, each sample value having either a first state or asecond state according to whether the input symbol exceeds a respectivethreshold level; and a decision circuit to generate a received datavalue, the decision circuit being operable in a first mode to select,based at least in part on a previously selected sample value, a samplevalue from the at least two sample values to be the received data value,the decision circuit further being operable in a second mode to selecteither a first of the at least two sample values or a second of the atleast two sample values to be the received data value, the decisioncircuit further being operable in a third mode to generate the receiveddata value having a most significant bit based at least in part on thestate of a first of the at least two sample values and a leastsignificant bit based at least in part on the state of a second of theat least two sample values.
 22. The receive circuit of claim 21, whereinthe compare circuit generates the at least two sample values based atleast in part on at least one previously received data value.
 23. Thereceive circuit of claim 21, wherein the decision circuit further beingoperable in the first mode to select, based at least in part on at leasttwo previously selected sample values, a sample value from the at leasttwo sample values to be the received data value.
 24. A method ofoperating a receive circuit apparatus comprising: generating at leasttwo sample values of an input symbol with sampling circuitry, eachsample value having either a first state or a second state according towhether the input symbol exceeds a respective threshold level; in afirst mode, selecting, based at least in part on a previously selectedsample value, a sample value from the at least two sample values to be areceived data value; and in a second mode, selecting a predetermined oneof the at least two sample values to be the received data value.
 25. Amethod of operating a receive circuit apparatus comprising: generatingat least two sample values of an input symbol with sampling circuitry,each sample value having either a first state or a second stateaccording to whether the input symbol exceeds a respective thresholdlevel; in a first mode, selecting, based at least in part on apreviously selected sample value, a sample value from the at least twosample values to be a received data value; and in a second mode,generating the received data value having a most significant bit basedat least in part on the state of a first of the at least two samplevalues and a least significant bit based at least in part on the stateof a second of the at least two sample values.
 26. A method of operatinga receive circuit apparatus comprising: generating at least two samplevalues of an input symbol with sampling circuitry, each sample valuehaving either a first state or a second state according to whether theinput symbol exceeds a respective threshold level; in a first mode,selecting, based at least in part on a previously selected sample value,a sample value from the at least two sample values to be a received datavalue; in a second mode, selecting either a first of the at least twosample values or a second of the at least two sample values to be thereceived data value; and in a third mode, generating the received datavalue having a most significant bit based at least in part on the stateof a first of the at least two sample values and a least significant bitbased at least in part on the state of a second of the at least twosample values.
 27. A receiver circuit comprising: a plurality ofsampling circuits, each of the plurality of sampling circuits to samplea signal and generate a sample value that indicates whether the signalexceeds a respective threshold level; a select circuit to receive thesample values from the plurality of sampling circuits and to select,based at least in part upon at least one previously selected samplevalue, one of the sample values as a currently selected sample value;and a threshold generation circuit to adaptively generate the thresholdlevels based on a determination of data-conveying levels of the signaland to generate, based on the first and second data-level thresholds, adecision threshold for distinguishing between data-conveying levels ofthe signal, the decision threshold being compared with the signal withinat least one of the plurality of sampling circuits to generate thesample value.
 28. The receiver circuit of claim 27 wherein the circuitryto adaptively generate the threshold levels based on a determination ofdata-conveying levels of the signal comprises circuitry to iterativelyadjust a first data-level threshold to match a level of the signal thatcorresponds to a first pattern of bits conveyed sequentially in thesignal and to iteratively adjust a second data-level threshold to matcha level of the signal that corresponds to a second pattern of bitsconveyed sequentially in the signal.